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PERLSUB(1)                            Perl Programmers Reference Guide                            PERLSUB(1)



NAME
       perlsub - Perl subroutines

SYNOPSIS
       To declare subroutines:

           sub NAME;                     # A "forward" declaration.
           sub NAME(PROTO);              #  ditto, but with prototypes
           sub NAME : ATTRS;             #  with attributes
           sub NAME(PROTO) : ATTRS;      #  with attributes and prototypes

           sub NAME BLOCK                # A declaration and a definition.
           sub NAME(PROTO) BLOCK         #  ditto, but with prototypes
           sub NAME : ATTRS BLOCK        #  with attributes
           sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes

       To define an anonymous subroutine at runtime:

           $subref = sub BLOCK;                 # no proto
           $subref = sub (PROTO) BLOCK;         # with proto
           $subref = sub : ATTRS BLOCK;         # with attributes
           $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes

       To import subroutines:

           use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

           NAME(LIST);    # & is optional with parentheses.
           NAME LIST;     # Parentheses optional if predeclared/imported.
           &NAME(LIST);   # Circumvent prototypes.
           &NAME;         # Makes current @_ visible to called subroutine.

DESCRIPTION
       Like many languages, Perl provides for user-defined subroutines.  These may be located anywhere in
       the main program, loaded in from other files via the "do", "require", or "use" keywords, or generated
       on the fly using "eval" or anonymous subroutines.  You can even call a function indirectly using a
       variable containing its name or a CODE reference.

       The Perl model for function call and return values is simple: all functions are passed as parameters
       one single flat list of scalars, and all functions likewise return to their caller one single flat
       list of scalars.  Any arrays or hashes in these call and return lists will collapse, losing their
       identities--but you may always use pass-by-reference instead to avoid this.  Both call and return
       lists may contain as many or as few scalar elements as you'd like.  (Often a function without an
       explicit return statement is called a subroutine, but there's really no difference from Perl's
       perspective.)

       Any arguments passed in show up in the array @_.  Therefore, if you called a function with two
       arguments, those would be stored in $_[0] and $_[1].  The array @_ is a local array, but its elements
       are aliases for the actual scalar parameters.  In particular, if an element $_[0] is updated, the
       corresponding argument is updated (or an error occurs if it is not updatable).  If an argument is an
       array or hash element which did not exist when the function was called, that element is created only
       when (and if) it is modified or a reference to it is taken.  (Some earlier versions of Perl created
       the element whether or not the element was assigned to.)  Assigning to the whole array @_ removes
       that aliasing, and does not update any arguments.

       A "return" statement may be used to exit a subroutine, optionally specifying the returned value,
       which will be evaluated in the appropriate context (list, scalar, or void) depending on the context
       of the subroutine call.  If you specify no return value, the subroutine returns an empty list in list
       context, the undefined value in scalar context, or nothing in void context.  If you return one or
       more aggregates (arrays and hashes), these will be flattened together into one large
       indistinguishable list.

       If no "return" is found and if the last statement is an expression, its value is returned. If the
       last statement is a loop control structure like a "foreach" or a "while", the returned value is
       unspecified. The empty sub returns the empty list.

       Perl does not have named formal parameters.  In practice all you do is assign to a "my()" list of
       these.  Variables that aren't declared to be private are global variables.  For gory details on
       creating private variables, see "Private Variables via my()" and "Temporary Values via local()".  To
       create protected environments for a set of functions in a separate package (and probably a separate
       file), see "Packages" in perlmod.

       Example:

           sub max {
               my $max = shift(@_);
               foreach $foo (@_) {
                   $max = $foo if $max < $foo;
               }
               return $max;
           }
           $bestday = max($mon,$tue,$wed,$thu,$fri);

       Example:

           # get a line, combining continuation lines
           #  that start with whitespace

           sub get_line {
               $thisline = $lookahead;  # global variables!
               LINE: while (defined($lookahead = <STDIN>)) {
                   if ($lookahead =~ /^[ \t]/) {
                       $thisline .= $lookahead;
                   }
                   else {
                       last LINE;
                   }
               }
               return $thisline;
           }

           $lookahead = <STDIN>;       # get first line
           while (defined($line = get_line())) {
               ...
           }

       Assigning to a list of private variables to name your arguments:

           sub maybeset {
               my($key, $value) = @_;
               $Foo{$key} = $value unless $Foo{$key};
           }

       Because the assignment copies the values, this also has the effect of turning call-by-reference into
       call-by-value.  Otherwise a function is free to do in-place modifications of @_ and change its
       caller's values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }
           }

       You aren't allowed to modify constants in this way, of course.  If an argument were actually literal
       and you tried to change it, you'd take a (presumably fatal) exception.   For example, this won't
       work:

           upcase_in("frederick");

       It would be much safer if the "upcase_in()" function were written to return a copy of its parameters
       instead of changing them in place:

           ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
           sub upcase {
               return unless defined wantarray;  # void context, do nothing
               my @parms = @_;
               for (@parms) { tr/a-z/A-Z/ }
               return wantarray ? @parms : $parms[0];
           }

       Notice how this (unprototyped) function doesn't care whether it was passed real scalars or arrays.
       Perl sees all arguments as one big, long, flat parameter list in @_.  This is one area where Perl's
       simple argument-passing style shines.  The "upcase()" function would work perfectly well without
       changing the "upcase()" definition even if we fed it things like this:

           @newlist   = upcase(@list1, @list2);
           @newlist   = upcase( split /:/, $var );

       Do not, however, be tempted to do this:

           (@a, @b)   = upcase(@list1, @list2);

       Like the flattened incoming parameter list, the return list is also flattened on return.  So all you
       have managed to do here is stored everything in @a and made @b empty.  See "Pass by Reference" for
       alternatives.

       A subroutine may be called using an explicit "&" prefix.  The "&" is optional in modern Perl, as are
       parentheses if the subroutine has been predeclared.  The "&" is not optional when just naming the
       subroutine, such as when it's used as an argument to defined() or undef().  Nor is it optional when
       you want to do an indirect subroutine call with a subroutine name or reference using the "&$subref()"
       or "&{$subref}()" constructs, although the "$subref->()" notation solves that problem.  See perlref
       for more about all that.

       Subroutines may be called recursively.  If a subroutine is called using the "&" form, the argument
       list is optional, and if omitted, no @_ array is set up for the subroutine: the @_ array at the time
       of the call is visible to subroutine instead.  This is an efficiency mechanism that new users may
       wish to avoid.

           &foo(1,2,3);        # pass three arguments
           foo(1,2,3);         # the same

           foo();              # pass a null list
           &foo();             # the same

           &foo;               # foo() get current args, like foo(@_) !!
           foo;                # like foo() IFF sub foo predeclared, else "foo"

       Not only does the "&" form make the argument list optional, it also disables any prototype checking
       on arguments you do provide.  This is partly for historical reasons, and partly for having a
       convenient way to cheat if you know what you're doing.  See "Prototypes" below.

       Since Perl 5.16.0, the "__SUB__" token is available under "use feature 'current_sub'" and "use
       5.16.0".  It will evaluate to a reference to the currently-running sub, which allows for recursive
       calls without knowing your subroutine's name.

           use 5.16.0;
           my $factorial = sub {
             my ($x) = @_;
             return 1 if $x == 1;
             return($x * __SUB__->( $x - 1 ) );
           };

       Subroutines whose names are in all upper case are reserved to the Perl core, as are modules whose
       names are in all lower case.  A subroutine in all capitals is a loosely-held convention meaning it
       will be called indirectly by the run-time system itself, usually due to a triggered event.
       Subroutines that do special, pre-defined things include "AUTOLOAD", "CLONE", "DESTROY" plus all
       functions mentioned in perltie and PerlIO::via.

       The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END" subroutines are not so much subroutines as named
       special code blocks, of which you can have more than one in a package, and which you can not call
       explicitly.  See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod

   Private Variables via my()
       Synopsis:

           my $foo;            # declare $foo lexically local
           my (@wid, %get);    # declare list of variables local
           my $foo = "flurp";  # declare $foo lexical, and init it
           my @oof = @bar;     # declare @oof lexical, and init it
           my $x : Foo = $y;   # similar, with an attribute applied

       WARNING: The use of attribute lists on "my" declarations is still evolving.  The current semantics
       and interface are subject to change.  See attributes and Attribute::Handlers.

       The "my" operator declares the listed variables to be lexically confined to the enclosing block,
       conditional ("if/unless/elsif/else"), loop ("for/foreach/while/until/continue"), subroutine, "eval",
       or "do/require/use"'d file.  If more than one value is listed, the list must be placed in
       parentheses.  All listed elements must be legal lvalues.  Only alphanumeric identifiers may be
       lexically scoped--magical built-ins like $/ must currently be "local"ized with "local" instead.

       Unlike dynamic variables created by the "local" operator, lexical variables declared with "my" are
       totally hidden from the outside world, including any called subroutines.  This is true if it's the
       same subroutine called from itself or elsewhere--every call gets its own copy.

       This doesn't mean that a "my" variable declared in a statically enclosing lexical scope would be
       invisible.  Only dynamic scopes are cut off.   For example, the "bumpx()" function below has access
       to the lexical $x variable because both the "my" and the "sub" occurred at the same scope, presumably
       file scope.

           my $x = 10;
           sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the scope it is being evaluated in, so long as the
       names aren't hidden by declarations within the "eval()" itself.  See perlref.

       The parameter list to my() may be assigned to if desired, which allows you to initialize your
       variables.  (If no initializer is given for a particular variable, it is created with the undefined
       value.)  Commonly this is used to name input parameters to a subroutine.  Examples:

           $arg = "fred";        # "global" variable
           $n = cube_root(27);
           print "$arg thinks the root is $n\n";
        fred thinks the root is 3

           sub cube_root {
               my $arg = shift;  # name doesn't matter
               $arg **= 1/3;
               return $arg;
           }

       The "my" is simply a modifier on something you might assign to.  So when you do assign to variables
       in its argument list, "my" doesn't change whether those variables are viewed as a scalar or an array.
       So

           my ($foo) = <STDIN>;                # WRONG?
           my @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           my $foo = <STDIN>;

       supplies a scalar context.  But the following declares only one variable:

           my $foo, $bar = 1;                  # WRONG

       That has the same effect as

           my $foo;
           $bar = 1;

       The declared variable is not introduced (is not visible) until after the current statement.  Thus,

           my $x = $x;

       can be used to initialize a new $x with the value of the old $x, and the expression

           my $x = 123 and $x == 123

       is false unless the old $x happened to have the value 123.

       Lexical scopes of control structures are not bounded precisely by the braces that delimit their
       controlled blocks; control expressions are part of that scope, too.  Thus in the loop

           while (my $line = <>) {
               $line = lc $line;
           } continue {
               print $line;
           }

       the scope of $line extends from its declaration throughout the rest of the loop construct (including
       the "continue" clause), but not beyond it.  Similarly, in the conditional

           if ((my $answer = <STDIN>) =~ /^yes$/i) {
               user_agrees();
           } elsif ($answer =~ /^no$/i) {
               user_disagrees();
           } else {
               chomp $answer;
               die "'$answer' is neither 'yes' nor 'no'";
           }

       the scope of $answer extends from its declaration through the rest of that conditional, including any
       "elsif" and "else" clauses, but not beyond it.  See "Simple Statements" in perlsyn for information on
       the scope of variables in statements with modifiers.

       The "foreach" loop defaults to scoping its index variable dynamically in the manner of "local".
       However, if the index variable is prefixed with the keyword "my", or if there is already a lexical by
       that name in scope, then a new lexical is created instead.  Thus in the loop

           for my $i (1, 2, 3) {
               some_function();
           }

       the scope of $i extends to the end of the loop, but not beyond it, rendering the value of $i
       inaccessible within "some_function()".

       Some users may wish to encourage the use of lexically scoped variables.  As an aid to catching
       implicit uses to package variables, which are always global, if you say

           use strict 'vars';

       then any variable mentioned from there to the end of the enclosing block must either refer to a
       lexical variable, be predeclared via "our" or "use vars", or else must be fully qualified with the
       package name.  A compilation error results otherwise.  An inner block may countermand this with "no
       strict 'vars'".

       A "my" has both a compile-time and a run-time effect.  At compile time, the compiler takes notice of
       it.  The principal usefulness of this is to quiet "use strict 'vars'", but it is also essential for
       generation of closures as detailed in perlref.  Actual initialization is delayed until run time,
       though, so it gets executed at the appropriate time, such as each time through a loop, for example.

       Variables declared with "my" are not part of any package and are therefore never fully qualified with
       the package name.  In particular, you're not allowed to try to make a package variable (or other
       global) lexical:

           my $pack::var;      # ERROR!  Illegal syntax

       In fact, a dynamic variable (also known as package or global variables) are still accessible using
       the fully qualified "::" notation even while a lexical of the same name is also visible:

           package main;
           local $x = 10;
           my    $x = 20;
           print "$x and $::x\n";

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of a file to hide any such identifiers from the
       world outside that file.  This is similar in spirit to C's static variables when they are used at the
       file level.  To do this with a subroutine requires the use of a closure (an anonymous function that
       accesses enclosing lexicals).  If you want to create a private subroutine that cannot be called from
       outside that block, it can declare a lexical variable containing an anonymous sub reference:

           my $secret_version = '1.001-beta';
           my $secret_sub = sub { print $secret_version };
           &$secret_sub();

       As long as the reference is never returned by any function within the module, no outside module can
       see the subroutine, because its name is not in any package's symbol table.  Remember that it's not
       REALLY called $some_pack::secret_version or anything; it's just $secret_version, unqualified and
       unqualifiable.

       This does not work with object methods, however; all object methods have to be in the symbol table of
       some package to be found.  See "Function Templates" in perlref for something of a work-around to
       this.

   Persistent Private Variables
       There are two ways to build persistent private variables in Perl 5.10.  First, you can simply use the
       "state" feature. Or, you can use closures, if you want to stay compatible with releases older than
       5.10.

       Persistent variables via state()

       Beginning with Perl 5.9.4, you can declare variables with the "state" keyword in place of "my".  For
       that to work, though, you must have enabled that feature beforehand, either by using the "feature"
       pragma, or by using "-E" on one-liners (see feature).  Beginning with Perl 5.16, the "CORE::state"
       form does not require the "feature" pragma.

       For example, the following code maintains a private counter, incremented each time the
       gimme_another() function is called:

           use feature 'state';
           sub gimme_another { state $x; return ++$x }

       Also, since $x is lexical, it can't be reached or modified by any Perl code outside.

       When combined with variable declaration, simple scalar assignment to "state" variables (as in "state
       $x = 42") is executed only the first time.  When such statements are evaluated subsequent times, the
       assignment is ignored.  The behavior of this sort of assignment to non-scalar variables is undefined.

       Persistent variables with closures

       Just because a lexical variable is lexically (also called statically) scoped to its enclosing block,
       "eval", or "do" FILE, this doesn't mean that within a function it works like a C static.  It normally
       works more like a C auto, but with implicit garbage collection.

       Unlike local variables in C or C++, Perl's lexical variables don't necessarily get recycled just
       because their scope has exited.  If something more permanent is still aware of the lexical, it will
       stick around.  So long as something else references a lexical, that lexical won't be freed--which is
       as it should be.  You wouldn't want memory being free until you were done using it, or kept around
       once you were done.  Automatic garbage collection takes care of this for you.

       This means that you can pass back or save away references to lexical variables, whereas to return a
       pointer to a C auto is a grave error.  It also gives us a way to simulate C's function statics.
       Here's a mechanism for giving a function private variables with both lexical scoping and a static
       lifetime.  If you do want to create something like C's static variables, just enclose the whole
       function in an extra block, and put the static variable outside the function but in the block.

           {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }
           # $secret_val now becomes unreachable by the outside
           # world, but retains its value between calls to gimme_another

       If this function is being sourced in from a separate file via "require" or "use", then this is
       probably just fine.  If it's all in the main program, you'll need to arrange for the "my" to be
       executed early, either by putting the whole block above your main program, or more likely, placing
       merely a "BEGIN" code block around it to make sure it gets executed before your program starts to
       run:

           BEGIN {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }

       See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the special triggered code blocks,
       "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END".

       If declared at the outermost scope (the file scope), then lexicals work somewhat like C's file
       statics.  They are available to all functions in that same file declared below them, but are
       inaccessible from outside that file.  This strategy is sometimes used in modules to create private
       variables that the whole module can see.

   Temporary Values via local()
       WARNING: In general, you should be using "my" instead of "local", because it's faster and safer.
       Exceptions to this include the global punctuation variables, global filehandles and formats, and
       direct manipulation of the Perl symbol table itself.  "local" is mostly used when the current value
       of a variable must be visible to called subroutines.

       Synopsis:

           # localization of values

           local $foo;                 # make $foo dynamically local
           local (@wid, %get);         # make list of variables local
           local $foo = "flurp";       # make $foo dynamic, and init it
           local @oof = @bar;          # make @oof dynamic, and init it

           local $hash{key} = "val";   # sets a local value for this hash entry
           delete local $hash{key};    # delete this entry for the current block
           local ($cond ? $v1 : $v2);  # several types of lvalues support
                                       # localization

           # localization of symbols

           local *FH;                  # localize $FH, @FH, %FH, &FH  ...
           local *merlyn = *randal;    # now $merlyn is really $randal, plus
                                       #     @merlyn is really @randal, etc
           local *merlyn = 'randal';   # SAME THING: promote 'randal' to *randal
           local *merlyn = \$randal;   # just alias $merlyn, not @merlyn etc

       A "local" modifies its listed variables to be "local" to the enclosing block, "eval", or "do
       FILE"--and to any subroutine called from within that block.  A "local" just gives temporary values to
       global (meaning package) variables.  It does not create a local variable.  This is known as dynamic
       scoping.  Lexical scoping is done with "my", which works more like C's auto declarations.

       Some types of lvalues can be localized as well: hash and array elements and slices, conditionals
       (provided that their result is always localizable), and symbolic references.  As for simple
       variables, this creates new, dynamically scoped values.

       If more than one variable or expression is given to "local", they must be placed in parentheses.
       This operator works by saving the current values of those variables in its argument list on a hidden
       stack and restoring them upon exiting the block, subroutine, or eval.  This means that called
       subroutines can also reference the local variable, but not the global one.  The argument list may be
       assigned to if desired, which allows you to initialize your local variables.  (If no initializer is
       given for a particular variable, it is created with an undefined value.)

       Because "local" is a run-time operator, it gets executed each time through a loop.  Consequently,
       it's more efficient to localize your variables outside the loop.

       Grammatical note on local()

       A "local" is simply a modifier on an lvalue expression.  When you assign to a "local"ized variable,
       the "local" doesn't change whether its list is viewed as a scalar or an array.  So

           local($foo) = <STDIN>;
           local @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           local $foo = <STDIN>;

       supplies a scalar context.

       Localization of special variables

       If you localize a special variable, you'll be giving a new value to it, but its magic won't go away.
       That means that all side-effects related to this magic still work with the localized value.

       This feature allows code like this to work :

           # Read the whole contents of FILE in $slurp
           { local $/ = undef; $slurp = <FILE>; }

       Note, however, that this restricts localization of some values ; for example, the following statement
       dies, as of perl 5.9.0, with an error Modification of a read-only value attempted, because the $1
       variable is magical and read-only :

           local $1 = 2;

       One exception is the default scalar variable: starting with perl 5.14 "local($_)" will always strip
       all magic from $_, to make it possible to safely reuse $_ in a subroutine.

       WARNING: Localization of tied arrays and hashes does not currently work as described.  This will be
       fixed in a future release of Perl; in the meantime, avoid code that relies on any particular
       behaviour of localising tied arrays or hashes (localising individual elements is still okay).  See
       "Localising Tied Arrays and Hashes Is Broken" in perl58delta for more details.

       Localization of globs

       The construct

           local *name;

       creates a whole new symbol table entry for the glob "name" in the current package.  That means that
       all variables in its glob slot ($name, @name, %name, &name, and the "name" filehandle) are
       dynamically reset.

       This implies, among other things, that any magic eventually carried by those variables is locally
       lost.  In other words, saying "local */" will not have any effect on the internal value of the input
       record separator.

       Localization of elements of composite types

       It's also worth taking a moment to explain what happens when you "local"ize a member of a composite
       type (i.e. an array or hash element).  In this case, the element is "local"ized by name. This means
       that when the scope of the "local()" ends, the saved value will be restored to the hash element whose
       key was named in the "local()", or the array element whose index was named in the "local()".  If that
       element was deleted while the "local()" was in effect (e.g. by a "delete()" from a hash or a
       "shift()" of an array), it will spring back into existence, possibly extending an array and filling
       in the skipped elements with "undef".  For instance, if you say

           %hash = ( 'This' => 'is', 'a' => 'test' );
           @ary  = ( 0..5 );
           {
                local($ary[5]) = 6;
                local($hash{'a'}) = 'drill';
                while (my $e = pop(@ary)) {
                    print "$e . . .\n";
                    last unless $e > 3;
                }
                if (@ary) {
                    $hash{'only a'} = 'test';
                    delete $hash{'a'};
                }
           }
           print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
           print "The array has ",scalar(@ary)," elements: ",
                 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

           6 . . .
           4 . . .
           3 . . .
           This is a test only a test.
           The array has 6 elements: 0, 1, 2, undef, undef, 5

       The behavior of local() on non-existent members of composite types is subject to change in future.

       Localized deletion of elements of composite types

       You can use the "delete local $array[$idx]" and "delete local $hash{key}" constructs to delete a
       composite type entry for the current block and restore it when it ends. They return the array/hash
       value before the localization, which means that they are respectively equivalent to

           do {
               my $val = $array[$idx];
               local  $array[$idx];
               delete $array[$idx];
               $val
           }

       and

           do {
               my $val = $hash{key};
               local  $hash{key};
               delete $hash{key};
               $val
           }

       except that for those the "local" is scoped to the "do" block. Slices are also accepted.

           my %hash = (
            a => [ 7, 8, 9 ],
            b => 1,
           )

           {
            my $a = delete local $hash{a};
            # $a is [ 7, 8, 9 ]
            # %hash is (b => 1)

            {
             my @nums = delete local @$a[0, 2]
             # @nums is (7, 9)
             # $a is [ undef, 8 ]

             $a[0] = 999; # will be erased when the scope ends
            }
            # $a is back to [ 7, 8, 9 ]

           }
           # %hash is back to its original state

   Lvalue subroutines
       WARNING: Lvalue subroutines are still experimental and the implementation may change in future
       versions of Perl.

       It is possible to return a modifiable value from a subroutine.  To do this, you have to declare the
       subroutine to return an lvalue.

           my $val;
           sub canmod : lvalue {
               $val;  # or:  return $val;
           }
           sub nomod {
               $val;
           }

           canmod() = 5;   # assigns to $val
           nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the right-hand side of assignment is determined as
       if the subroutine call is replaced by a scalar. For example, consider:

           data(2,3) = get_data(3,4);

       Both subroutines here are called in a scalar context, while in:

           (data(2,3)) = get_data(3,4);

       and in:

           (data(2),data(3)) = get_data(3,4);

       all the subroutines are called in a list context.

       Lvalue subroutines are EXPERIMENTAL
           They appear to be convenient, but there is at least one reason to be circumspect.

           They violate encapsulation.  A normal mutator can check the supplied argument before setting the
           attribute it is protecting, an lvalue subroutine never gets that chance.  Consider;

               my $some_array_ref = [];    # protected by mutators ??

               sub set_arr {               # normal mutator
                   my $val = shift;
                   die("expected array, you supplied ", ref $val)
                      unless ref $val eq 'ARRAY';
                   $some_array_ref = $val;
               }
               sub set_arr_lv : lvalue {   # lvalue mutator
                   $some_array_ref;
               }

               # set_arr_lv cannot stop this !
               set_arr_lv() = { a => 1 };

   Passing Symbol Table Entries (typeglobs)
       WARNING: The mechanism described in this section was originally the only way to simulate pass-by-reference pass-byreference
       reference in older versions of Perl.  While it still works fine in modern versions, the new reference
       mechanism is generally easier to work with.  See below.

       Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so
       that the subroutine can modify the global copy of it rather than working with a local copy.  In perl
       you can refer to all objects of a particular name by prefixing the name with a star: *foo.  This is
       often known as a "typeglob", because the star on the front can be thought of as a wildcard match for
       all the funny prefix characters on variables and subroutines and such.

       When evaluated, the typeglob produces a scalar value that represents all the objects of that name,
       including any filehandle, format, or subroutine.  When assigned to, it causes the name mentioned to
       refer to whatever "*" value was assigned to it.  Example:

           sub doubleary {
               local(*someary) = @_;
               foreach $elem (@someary) {
                   $elem *= 2;
               }
           }
           doubleary(*foo);
           doubleary(*bar);

       Scalars are already passed by reference, so you can modify scalar arguments without using this
       mechanism by referring explicitly to $_[0] etc.  You can modify all the elements of an array by
       passing all the elements as scalars, but you have to use the "*" mechanism (or the equivalent
       reference mechanism) to "push", "pop", or change the size of an array.  It will certainly be faster
       to pass the typeglob (or reference).

       Even if you don't want to modify an array, this mechanism is useful for passing multiple arrays in a
       single LIST, because normally the LIST mechanism will merge all the array values so that you can't
       extract out the individual arrays.  For more on typeglobs, see "Typeglobs and Filehandles" in
       perldata.

   When to Still Use local()
       Despite the existence of "my", there are still three places where the "local" operator still shines.
       In fact, in these three places, you must use "local" instead of "my".

       1.  You need to give a global variable a temporary value, especially $_.

           The global variables, like @ARGV or the punctuation variables, must be "local"ized with
           "local()".  This block reads in /etc/motd, and splits it up into chunks separated by lines of
           equal signs, which are placed in @Fields.

               {
                   local @ARGV = ("/etc/motd");
                   local $/ = undef;
                   local $_ = <>;
                   @Fields = split /^\s*=+\s*$/;
               }

           It particular, it's important to "local"ize $_ in any routine that assigns to it.  Look out for
           implicit assignments in "while" conditionals.

       2.  You need to create a local file or directory handle or a local function.

           A function that needs a filehandle of its own must use "local()" on a complete typeglob.   This
           can be used to create new symbol table entries:

               sub ioqueue {
                   local  (*READER, *WRITER);    # not my!
                   pipe    (READER,  WRITER)     or die "pipe: $!";
                   return (*READER, *WRITER);
               }
               ($head, $tail) = ioqueue();

           See the Symbol module for a way to create anonymous symbol table entries.

           Because assignment of a reference to a typeglob creates an alias, this can be used to create what
           is effectively a local function, or at least, a local alias.

               {
                   local *grow = \&shrink; # only until this block exits
                   grow();                 # really calls shrink()
                   move();                 # if move() grow()s, it shrink()s too
               }
               grow();                     # get the real grow() again

           See "Function Templates" in perlref for more about manipulating functions by name in this way.

       3.  You want to temporarily change just one element of an array or hash.

           You can "local"ize just one element of an aggregate.  Usually this is done on dynamics:

               {
                   local $SIG{INT} = 'IGNORE';
                   funct();                            # uninterruptible
               }
               # interruptibility automatically restored here

           But it also works on lexically declared aggregates.  Prior to 5.005, this operation could on
           occasion misbehave.

   Pass by Reference
       If you want to pass more than one array or hash into a function--or return them from it--and have
       them maintain their integrity, then you're going to have to use an explicit pass-by-reference.
       Before you do that, you need to understand references as detailed in perlref.  This section may not
       make much sense to you otherwise.

       Here are a few simple examples.  First, let's pass in several arrays to a function and have it "pop"
       all of then, returning a new list of all their former last elements:

           @tailings = popmany ( \@a, \@b, \@c, \@d );

           sub popmany {
               my $aref;
               my @retlist = ();
               foreach $aref ( @_ ) {
                   push @retlist, pop @$aref;
               }
               return @retlist;
           }

       Here's how you might write a function that returns a list of keys occurring in all the hashes passed
       to it:

           @common = inter( \%foo, \%bar, \%joe );
           sub inter {
               my ($k, $href, %seen); # locals
               foreach $href (@_) {
                   while ( $k = each %$href ) {
                       $seen{$k}++;
                   }
               }
               return grep { $seen{$_} == @_ } keys %seen;
           }

       So far, we're using just the normal list return mechanism.  What happens if you want to pass or
       return a hash?  Well, if you're using only one of them, or you don't mind them concatenating, then
       the normal calling convention is ok, although a little expensive.

       Where people get into trouble is here:

           (@a, @b) = func(@c, @d);
       or
           (%a, %b) = func(%c, %d);

       That syntax simply won't work.  It sets just @a or %a and clears the @b or %b.  Plus the function
       didn't get passed into two separate arrays or hashes: it got one long list in @_, as always.

       If you can arrange for everyone to deal with this through references, it's cleaner code, although not
       so nice to look at.  Here's a function that takes two array references as arguments, returning the
       two array elements in order of how many elements they have in them:

           ($aref, $bref) = func(\@c, \@d);
           print "@$aref has more than @$bref\n";
           sub func {
               my ($cref, $dref) = @_;
               if (@$cref > @$dref) {
                   return ($cref, $dref);
               } else {
                   return ($dref, $cref);
               }
           }

       It turns out that you can actually do this also:

           (*a, *b) = func(\@c, \@d);
           print "@a has more than @b\n";
           sub func {
               local (*c, *d) = @_;
               if (@c > @d) {
                   return (\@c, \@d);
               } else {
                   return (\@d, \@c);
               }
           }

       Here we're using the typeglobs to do symbol table aliasing.  It's a tad subtle, though, and also
       won't work if you're using "my" variables, because only globals (even in disguise as "local"s) are in
       the symbol table.

       If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but
       typeglobs references work, too.  For example:

           splutter(\*STDOUT);
           sub splutter {
               my $fh = shift;
               print $fh "her um well a hmmm\n";
           }

           $rec = get_rec(\*STDIN);
           sub get_rec {
               my $fh = shift;
               return scalar <$fh>;
           }

       If you're planning on generating new filehandles, you could do this.  Notice to pass back just the
       bare *FH, not its reference.

           sub openit {
               my $path = shift;
               local *FH;
               return open (FH, $path) ? *FH : undef;
           }

   Prototypes
       Perl supports a very limited kind of compile-time argument checking using function prototyping.  If
       you declare

           sub mypush (+@)

       then "mypush()" takes arguments exactly like "push()" does.  The function declaration must be visible
       at compile time.  The prototype affects only interpretation of new-style calls to the function, where
       new-style is defined as not using the "&" character.  In other words, if you call it like a built-in
       function, then it behaves like a built-in function.  If you call it like an old-fashioned subroutine,
       then it behaves like an old-fashioned subroutine.  It naturally falls out from this rule that
       prototypes have no influence on subroutine references like "\&foo" or on indirect subroutine calls
       like "&{$subref}" or "$subref->()".

       Method calls are not influenced by prototypes either, because the function to be called is
       indeterminate at compile time, since the exact code called depends on inheritance.

       Because the intent of this feature is primarily to let you define subroutines that work like built-in
       functions, here are prototypes for some other functions that parse almost exactly like the
       corresponding built-in.

           Declared as                 Called as

           sub mylink ($$)          mylink $old, $new
           sub myvec ($$$)          myvec $var, $offset, 1
           sub myindex ($$;$)       myindex &getstring, "substr"
           sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) - $off, $off
           sub myreverse (@)        myreverse $a, $b, $c
           sub myjoin ($@)          myjoin ":", $a, $b, $c
           sub mypop (+)            mypop @array
           sub mysplice (+$$@)      mysplice @array, 0, 2, @pushme
           sub mykeys (+)           mykeys %{$hashref}
           sub myopen (*;$)         myopen HANDLE, $name
           sub mypipe (**)          mypipe READHANDLE, WRITEHANDLE
           sub mygrep (&@)          mygrep { /foo/ } $a, $b, $c
           sub myrand (;$)          myrand 42
           sub mytime ()            mytime

       Any backslashed prototype character represents an actual argument that must start with that character
       (optionally preceded by "my", "our" or "local"), with the exception of "$", which will accept any
       scalar lvalue expression, such as "$foo = 7" or "my_function()->[0]". The value passed as part of @_
       will be a reference to the actual argument given in the subroutine call, obtained by applying "\" to
       that argument.

       You can use the "\[]" backslash group notation to specify more than one allowed argument type. For
       example:

           sub myref (\[$@%&*])

       will allow calling myref() as

           myref $var
           myref @array
           myref %hash
           myref &sub
           myref *glob

       and the first argument of myref() will be a reference to a scalar, an array, a hash, a code, or a
       glob.

       Unbackslashed prototype characters have special meanings.  Any unbackslashed "@" or "%" eats all
       remaining arguments, and forces list context.  An argument represented by "$" forces scalar context.
       An "&" requires an anonymous subroutine, which, if passed as the first argument, does not require the
       "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept a bareword, constant, scalar expression, typeglob, or a
       reference to a typeglob in that slot.  The value will be available to the subroutine either as a
       simple scalar, or (in the latter two cases) as a reference to the typeglob.  If you wish to always
       convert such arguments to a typeglob reference, use Symbol::qualify_to_ref() as follows:

           use Symbol 'qualify_to_ref';

           sub foo (*) {
               my $fh = qualify_to_ref(shift, caller);
               ...
           }

       The "+" prototype is a special alternative to "$" that will act like "\[@%]" when given a literal
       array or hash variable, but will otherwise force scalar context on the argument.  This is useful for
       functions which should accept either a literal array or an array reference as the argument:

           sub mypush (+@) {
               my $aref = shift;
               die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
               push @$aref, @_;
           }

       When using the "+" prototype, your function must check that the argument is of an acceptable type.

       A semicolon (";") separates mandatory arguments from optional arguments.  It is redundant before "@"
       or "%", which gobble up everything else.

       As the last character of a prototype, or just before a semicolon, a "@" or a "%", you can use "_" in
       place of "$": if this argument is not provided, $_ will be used instead.

       Note how the last three examples in the table above are treated specially by the parser.  "mygrep()"
       is parsed as a true list operator, "myrand()" is parsed as a true unary operator with unary
       precedence the same as "rand()", and "mytime()" is truly without arguments, just like "time()".  That
       is, if you say

           mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it would be parsed without a prototype.  If
       you want to force a unary function to have the same precedence as a list operator, add ";" to the end
       of the prototype:

           sub mygetprotobynumber($;);
           mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)

       The interesting thing about "&" is that you can generate new syntax with it, provided it's in the
       initial position:

           sub try (&@) {
               my($try,$catch) = @_;
               eval { &$try };
               if ($@) {
                   local $_ = $@;
                   &$catch;
               }
           }
           sub catch (&) { $_[0] }

           try {
               die "phooey";
           } catch {
               /phooey/ and print "unphooey\n";
           };

       That prints "unphooey".  (Yes, there are still unresolved issues having to do with visibility of @_.
       I'm ignoring that question for the moment.  (But note that if we make @_ lexically scoped, those
       anonymous subroutines can act like closures... (Gee, is this sounding a little Lispish?  (Never
       mind.))))

       And here's a reimplementation of the Perl "grep" operator:

           sub mygrep (&@) {
               my $code = shift;
               my @result;
               foreach $_ (@_) {
                   push(@result, $_) if &$code;
               }
               @result;
           }

       Some folks would prefer full alphanumeric prototypes.  Alphanumerics have been intentionally left out
       of prototypes for the express purpose of someday in the future adding named, formal parameters.  The
       current mechanism's main goal is to let module writers provide better diagnostics for module users.
       Larry feels the notation quite understandable to Perl programmers, and that it will not intrude
       greatly upon the meat of the module, nor make it harder to read.  The line noise is visually
       encapsulated into a small pill that's easy to swallow.

       If you try to use an alphanumeric sequence in a prototype you will generate an optional warning -"Illegal warning"Illegal
       "Illegal character in prototype...".  Unfortunately earlier versions of Perl allowed the prototype to
       be used as long as its prefix was a valid prototype.  The warning may be upgraded to a fatal error in
       a future version of Perl once the majority of offending code is fixed.

       It's probably best to prototype new functions, not retrofit prototyping into older ones.  That's
       because you must be especially careful about silent impositions of differing list versus scalar
       contexts.  For example, if you decide that a function should take just one parameter, like this:

           sub func ($) {
               my $n = shift;
               print "you gave me $n\n";
           }

       and someone has been calling it with an array or expression returning a list:

           func(@foo);
           func( split /:/ );

       Then you've just supplied an automatic "scalar" in front of their argument, which can be more than a
       bit surprising.  The old @foo which used to hold one thing doesn't get passed in.  Instead, "func()"
       now gets passed in a 1; that is, the number of elements in @foo.  And the "split" gets called in
       scalar context so it starts scribbling on your @_ parameter list.  Ouch!

       This is all very powerful, of course, and should be used only in moderation to make the world a
       better place.

   Constant Functions
       Functions with a prototype of "()" are potential candidates for inlining.  If the result after
       optimization and constant folding is either a constant or a lexically-scoped scalar which has no
       other references, then it will be used in place of function calls made without "&".  Calls made using
       "&" are never inlined.  (See constant.pm for an easy way to declare most constants.)

       The following functions would all be inlined:

           sub pi ()           { 3.14159 }             # Not exact, but close.
           sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets,
                                                       # and it's inlined, too!
           sub ST_DEV ()       { 0 }
           sub ST_INO ()       { 1 }

           sub FLAG_FOO ()     { 1 << 8 }
           sub FLAG_BAR ()     { 1 << 9 }
           sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

           sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

           sub N () { int(OPT_BAZ) / 3 }

           sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }

       Be aware that these will not be inlined; as they contain inner scopes, the constant folding doesn't
       reduce them to a single constant:

           sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }

           sub baz_val () {
               if (OPT_BAZ) {
                   return 23;
               }
               else {
                   return 42;
               }
           }

       If you redefine a subroutine that was eligible for inlining, you'll get a warning by default.  (You
       can use this warning to tell whether or not a particular subroutine is considered constant.)  The
       warning is considered severe enough not to be affected by the -w switch (or its absence) because
       previously compiled invocations of the function will still be using the old value of the function.
       If you need to be able to redefine the subroutine, you need to ensure that it isn't inlined, either
       by dropping the "()" prototype (which changes calling semantics, so beware) or by thwarting the
       inlining mechanism in some other way, such as

           sub not_inlined () {
               23 if $];
           }

   Overriding Built-in Functions
       Many built-in functions may be overridden, though this should be tried only occasionally and for good
       reason.  Typically this might be done by a package attempting to emulate missing built-in
       functionality on a non-Unix system.

       Overriding may be done only by importing the name from a module at compile time--ordinary
       predeclaration isn't good enough.  However, the "use subs" pragma lets you, in effect, predeclare
       subs via the import syntax, and these names may then override built-in ones:

           use subs 'chdir', 'chroot', 'chmod', 'chown';
           chdir $somewhere;
           sub chdir { ... }

       To unambiguously refer to the built-in form, precede the built-in name with the special package
       qualifier "CORE::".  For example, saying "CORE::open()" always refers to the built-in "open()", even
       if the current package has imported some other subroutine called "&open()" from elsewhere.  Even
       though it looks like a regular function call, it isn't: the CORE:: prefix in that case is part of
       Perl's syntax, and works for any keyword, regardless of what is in the CORE package.  Taking a
       reference to it, that is, "\&CORE::open", only works for some keywords.  See CORE.

       Library modules should not in general export built-in names like "open" or "chdir" as part of their
       default @EXPORT list, because these may sneak into someone else's namespace and change the semantics
       unexpectedly.  Instead, if the module adds that name to @EXPORT_OK, then it's possible for a user to
       import the name explicitly, but not implicitly.  That is, they could say

           use Module 'open';

       and it would import the "open" override.  But if they said

           use Module;

       they would get the default imports without overrides.

       The foregoing mechanism for overriding built-in is restricted, quite deliberately, to the package
       that requests the import.  There is a second method that is sometimes applicable when you wish to
       override a built-in everywhere, without regard to namespace boundaries.  This is achieved by
       importing a sub into the special namespace "CORE::GLOBAL::".  Here is an example that quite brazenly
       replaces the "glob" operator with something that understands regular expressions.

           package REGlob;
           require Exporter;
           @ISA = 'Exporter';
           @EXPORT_OK = 'glob';

           sub import {
               my $pkg = shift;
               return unless @_;
               my $sym = shift;
               my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
               $pkg->export($where, $sym, @_);
           }

           sub glob {
               my $pat = shift;
               my @got;
               if (opendir my $d, '.') {
                   @got = grep /$pat/, readdir $d;
                   closedir $d;
               }
               return @got;
           }
           1;

       And here's how it could be (ab)used:

           #use REGlob 'GLOBAL_glob';      # override glob() in ALL namespaces
           package Foo;
           use REGlob 'glob';              # override glob() in Foo:: only
           print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules

       The initial comment shows a contrived, even dangerous example.  By overriding "glob" globally, you
       would be forcing the new (and subversive) behavior for the "glob" operator for every namespace,
       without the complete cognizance or cooperation of the modules that own those namespaces.  Naturally,
       this should be done with extreme caution--if it must be done at all.

       The "REGlob" example above does not implement all the support needed to cleanly override perl's
       "glob" operator.  The built-in "glob" has different behaviors depending on whether it appears in a
       scalar or list context, but our "REGlob" doesn't.  Indeed, many perl built-in have such context
       sensitive behaviors, and these must be adequately supported by a properly written override.  For a
       fully functional example of overriding "glob", study the implementation of "File::DosGlob" in the
       standard library.

       When you override a built-in, your replacement should be consistent (if possible) with the built-in
       native syntax.  You can achieve this by using a suitable prototype.  To get the prototype of an
       overridable built-in, use the "prototype" function with an argument of "CORE::builtin_name" (see
       "prototype" in perlfunc).

       Note however that some built-ins can't have their syntax expressed by a prototype (such as "system"
       or "chomp").  If you override them you won't be able to fully mimic their original syntax.

       The built-ins "do", "require" and "glob" can also be overridden, but due to special magic, their
       original syntax is preserved, and you don't have to define a prototype for their replacements.  (You
       can't override the "do BLOCK" syntax, though).

       "require" has special additional dark magic: if you invoke your "require" replacement as "require
       Foo::Bar", it will actually receive the argument "Foo/Bar.pm" in @_.  See "require" in perlfunc.

       And, as you'll have noticed from the previous example, if you override "glob", the "<*>" glob
       operator is overridden as well.

       In a similar fashion, overriding the "readline" function also overrides the equivalent I/O operator
       "<FILEHANDLE>". Also, overriding "readpipe" also overrides the operators "``" and "qx//".

       Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.

   Autoloading
       If you call a subroutine that is undefined, you would ordinarily get an immediate, fatal error
       complaining that the subroutine doesn't exist.  (Likewise for subroutines being used as methods, when
       the method doesn't exist in any base class of the class's package.)  However, if an "AUTOLOAD"
       subroutine is defined in the package or packages used to locate the original subroutine, then that
       "AUTOLOAD" subroutine is called with the arguments that would have been passed to the original
       subroutine.  The fully qualified name of the original subroutine magically appears in the global
       $AUTOLOAD variable of the same package as the "AUTOLOAD" routine.  The name is not passed as an
       ordinary argument because, er, well, just because, that's why.  (As an exception, a method call to a
       nonexistent "import" or "unimport" method is just skipped instead.  Also, if the AUTOLOAD subroutine
       is an XSUB, there are other ways to retrieve the subroutine name.  See "Autoloading with XSUBs" in
       perlguts for details.)

       Many "AUTOLOAD" routines load in a definition for the requested subroutine using eval(), then execute
       that subroutine using a special form of goto() that erases the stack frame of the "AUTOLOAD" routine
       without a trace.  (See the source to the standard module documented in AutoLoader, for example.)  But
       an "AUTOLOAD" routine can also just emulate the routine and never define it.   For example, let's
       pretend that a function that wasn't defined should just invoke "system" with those arguments.  All
       you'd do is:

           sub AUTOLOAD {
               my $program = $AUTOLOAD;
               $program =~ s/.*:://;
               system($program, @_);
           }
           date();
           who('am', 'i');
           ls('-l');

       In fact, if you predeclare functions you want to call that way, you don't even need parentheses:

           use subs qw(date who ls);
           date;
           who "am", "i";
           ls '-l';

       A more complete example of this is the Shell module on CPAN, which can treat undefined subroutine
       calls as calls to external programs.

       Mechanisms are available to help modules writers split their modules into autoloadable files.  See
       the standard AutoLoader module described in AutoLoader and in AutoSplit, the standard SelfLoader
       modules in SelfLoader, and the document on adding C functions to Perl code in perlxs.

   Subroutine Attributes
       A subroutine declaration or definition may have a list of attributes associated with it.  If such an
       attribute list is present, it is broken up at space or colon boundaries and treated as though a "use
       attributes" had been seen.  See attributes for details about what attributes are currently supported.
       Unlike the limitation with the obsolescent "use attrs", the "sub : ATTRLIST" syntax works to
       associate the attributes with a pre-declaration, and not just with a subroutine definition.

       The attributes must be valid as simple identifier names (without any punctuation other than the '_'
       character).  They may have a parameter list appended, which is only checked for whether its
       parentheses ('(',')') nest properly.

       Examples of valid syntax (even though the attributes are unknown):

           sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
           sub plugh () : Ugly('\(") :Bad;
           sub xyzzy : _5x5 { ... }

       Examples of invalid syntax:

           sub fnord : switch(10,foo(); # ()-string not balanced
           sub snoid : Ugly('(');        # ()-string not balanced
           sub xyzzy : 5x5;              # "5x5" not a valid identifier
           sub plugh : Y2::north;        # "Y2::north" not a simple identifier
           sub snurt : foo + bar;        # "+" not a colon or space

       The attribute list is passed as a list of constant strings to the code which associates them with the
       subroutine.  In particular, the second example of valid syntax above currently looks like this in
       terms of how it's parsed and invoked:

           use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

       For further details on attribute lists and their manipulation, see attributes and
       Attribute::Handlers.

SEE ALSO
       See "Function Templates" in perlref for more about references and closures.  See perlxs if you'd like
       to learn about calling C subroutines from Perl.  See perlembed if you'd like to learn about calling
       Perl subroutines from C.  See perlmod to learn about bundling up your functions in separate files.
       See perlmodlib to learn what library modules come standard on your system.  See perlootut to learn
       how to make object method calls.



perl v5.16.2                                     2012-10-25                                       PERLSUB(1)