Perl 5.12.0 introduced
This feature lets a module author extend Perl by defining custom keywords, at
least as long as that module author knows
XS and how to construct
Perl 5.14.0 added many functions to the API that make custom keywords worthwhile (especially the ability to invoke the Perl parser recursively in order to parse a custom syntax with embedded Perl fragments).
So what can this be used for? In the following, I'm going to show you three modules I've written that make heavy use of custom keywords.