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NAME
SYNOPSIS
DESCRIPTION
FEATURE BUNDLES
IMPLICIT LOADING

#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.

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