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feature(3pm)                          Perl Programmers Reference Guide                          feature(3pm)



NAME
       feature - Perl pragma to enable new features

SYNOPSIS
           use feature qw(switch say);
           given ($foo) {
               when (1)          { say "\$foo == 1" }
               when ([2,3])      { say "\$foo == 2 || \$foo == 3" }
               when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
               when ($_ > 100)   { say "\$foo > 100" }
               default           { say "None of the above" }
           }

           use feature ':5.10'; # loads all features available in perl 5.10

DESCRIPTION
       It is usually impossible to add new syntax to Perl without breaking some existing programs. This
       pragma provides a way to minimize that risk. New syntactic constructs, or new semantic meanings to
       older constructs, can be enabled by "use feature 'foo'", and will be parsed only when the appropriate
       feature pragma is in scope.

   Lexical effect
       Like other pragmas ("use strict", for example), features have a lexical effect. "use feature qw(foo)"
       will only make the feature "foo" available from that point to the end of the enclosing block.

           {
               use feature 'say';
               say "say is available here";
           }
           print "But not here.\n";

   "no feature"
       Features can also be turned off by using "no feature "foo"". This too has lexical effect.

           use feature 'say';
           say "say is available here";
           {
               no feature 'say';
               print "But not here.\n";
           }
           say "Yet it is here.";

       "no feature" with no features specified will turn off all features.

   The 'switch' feature
       "use feature 'switch'" tells the compiler to enable the Perl 6 given/when construct.

       See "Switch statements" in perlsyn for details.

   The 'say' feature
       "use feature 'say'" tells the compiler to enable the Perl 6 "say" function.

       See "say" in perlfunc for details.

   the 'state' feature
       "use feature 'state'" tells the compiler to enable "state" variables.

       See "Persistent Private Variables" in perlsub for details.

   the 'unicode_strings' feature
       "use feature 'unicode_strings'" tells the compiler to treat all strings outside of "use locale" and
       "use bytes" as Unicode. It is available starting with Perl 5.11.3.

       See "The "Unicode Bug"" in perlunicode for details.

FEATURE BUNDLES
       It's possible to load a whole slew of features in one go, using a feature bundle. The name of a
       feature bundle is prefixed with a colon, to distinguish it from an actual feature. At present, the
       only feature bundle is "use feature ":5.10"" which is equivalent to "use feature qw(switch say
       state)".

       Specifying sub-versions such as the 0 in 5.10.0 in feature bundles has no effect: feature bundles are
       guaranteed to be the same for all sub-versions.

IMPLICIT LOADING
       There are two ways to load the "feature" pragma implicitly :

       •   By using the "-E" switch on the command-line instead of "-e". It enables all available features
           in the main compilation unit (that is, the one-liner.)

       •   By requiring explicitly a minimal Perl version number for your program, with the "use VERSION"
           construct, and when the version is higher than or equal to 5.10.0. That is,

               use 5.10.0;

           will do an implicit

               use feature ':5.10';

           and so on. Note how the trailing sub-version is automatically stripped from the version.

           But to avoid portability warnings (see "use" in perlfunc), you may prefer:

               use 5.010;

           with the same effect.



perl v5.12.5                                     2012-11-03                                     feature(3pm)