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

NAME
       feature - Perl pragma to enable new syntactic 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 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.

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
       bundles are "use feature ":5.10"" and "use feature ":5.10.0"", which
       both are equivalent to "use feature qw(switch say state)".

       In the forthcoming 5.10.X perl releases, "use feature ":5.10"" will be
       equivalent to the latest "use feature ":5.10.X"".

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 ':5.10.0';

	   and so on.

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

	       use 5.010;

	   with the same effect.

perl v5.10.0			  2007-12-18			    feature(3)
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