Perl 5.12.0 introduced pluggable keywords. 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 OP trees manually.

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.

Why is a raven like a writing-desk? (Lewis Carroll)

This is a copy of an article I wrote a long time ago. I'm putting it here to give it a more permanent home. Sorry for being off topic again!

Introduction

I'm glad you asked. The answer is surprisingly simple: almost everything. In other words, they have almost nothing in common. To understand why, we'll take a look at what they are and what operations they support.