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.
       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 :

       o   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.)

       o   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.