By default new features are not enabled in Perl mainly to retain backward compatibility. Starting with release 5.10, latest features have to be enabled explicitly before using them. Here are a few ways to enable Perl features.

1. Enable new features with the help of -E switch.

   • Introduced in v5.10, -E is similar to -e switch except that -E also enables all latest features. In other words, -E switch on the Perl command line enables the feature bundle for that version of Perl in the main compilation unit. (see #4 below for enabling feature bundles explicitly).

   eg.,

   % perl -e 'print $^V;'
   v5.16.3
   
   % perl -e "print q(Hello)"
   Hello% ⏎
   % perl -E "print q(Hello)"
   Hello% ⏎
   % perl -e "say q(Hello)"
   syntax error at -e line 1, near "say q(Hello)"
   Execution of -e aborted due to compilation errors.
   % perl -E "say q(Hello)"
   Hello
   %
   

2. Enable one or more features selectively with the help of use feature pragma.

   • use feature pragma minimizes the risk of breaking existing programs as the specified feature will only be enabled within the scope of the pragma.

   eg.,

   % cat -n enable_say.pl
        1  #!/bin/perl
        2  say("Hello")
   
   % ./enable_say.pl
   Undefined subroutine &main::say called at ./enable_say.pl line 2.
   
   % cat -n enable_say.pl
        1  #!/bin/perl
        2  use feature say;
        3  say("Hello")
   
   % ./enable_say.pl
   Hello
   

   To enable multiple features, spearate each one with spaces. eg., use feature qw(say state switch unicode_strings).

3. Enable all features available in the requested version with the help of use <VERSION> directive.

   • use <VERSION> disables any features not in the requested version's feature bundle. (see #4 below for enabling feature bundles explicitly). This is useful to check the current Perl version before using library modules that won't work with older versions of Perl.

   eg.,

   % cat -n enable_say.pl
        1  #!/bin/perl
        2  say("Hello")
   
   % ./enable_say.pl
   Undefined subroutine &main::say called at ./enable_say.pl line 2.
   
   % cat -n enable_say.pl
        1  #!/bin/perl
        2  use v5.10;
        3  say("Hello")
   
   % ./enable_say.pl
   Hello
   

4. Similar to use <VERSION> directive, feature bundles help load multiple features together. Any feature bundle can be enabled with the help of use feature ":<FEATURE_BUNDLE>" directive.

   The colon that prefixes the feature bundle differentiates the bundle from an actual feature.

   eg.,

   To enable say, state, switch, unicode_strings, unicode_eval, evalbytes, current_sub and fc features, enable feature bundle 5.16 as shown below.

   #!/bin/perl
   ..
   use feature ":5.16";
   ..
   

   Perl features available in various feature bundles are listed here.

Credit: various sources

Labels: Perl Enable+Features Feature+Bundles

Posted by: Giri Mandalika.    # 8:16 PM Tweet