Perl 5.12.1はマイナーなバグの修正やドキュメント変更、モジュールアップグレードなどが主な内容で、それ以外は完全に後方互換なリリースですので5.12系を使う場合はアップグレードをお勧めいたします。

詳しくはperl5121delta.podをご覧ください

This release cycle marks a change to a time-based release process.

Beginning with version 5.11.0, we make a new development release

of Perl available on the 20th of each month.  Each spring, we will

release a new stable version of Perl.  One month later, we will make a 

minor update to deal with any issues discovered after the initial ".0"

release. Future releases in the stable series will follow quarterly.

In contrast to releases of Perl, maintenance releases will contain fixes

for issues discovered after the .0 release, but will not include new

features or behavior.

    use 5.12.0;

    use strict;
    use feature ':5.12';

    use feature 'unicode_strings';
    use feature 'say';      # say()
    use feature 'switch'; # given...when
    use feature 'state';   # state変数

    use 5.12.0;
    $foo = 1;

    { # このスコープではstrictが有効
       use 5.12.0;
    }

    { # 違うスコープ
        $foo = 1;
    }

     sub foo {
         my $self = shift;
         ...
     }

      package MyModule;
      our $VERSION = 1.23;

      package MyModule 1.23;

      package MyModule;
      our $VERSION;
      BEGIN {
           $VERSION = version->parse(1.23);
      }