You should really check out the Catalyst framework for Perl if you want a system that allows you to create plugins that augment and add functionality in a modular and maintainable way.
You do realize that you just linked to a module I maintain, right :)
FWIW, Catalyst plugins have a lot of problems, but they have also proven useful to people trying to extend the framework. 99% of the time, people can get a maintainable chunk of code into their application without having to monkey patch or change the core. That is a Good Thing, but we can do better :)
Catalyst 5.8 will use Moose roles for plugins, so the safety problems of mixins should go away. The next problem to solve is how to reuse cross-sections of applications (models, views, controllers, templates, etc.) in a clean manner. I'm experimenting with Bread::Board (IoC) in a new framework called Kiiteh. If Kiiteh pans out, it will be Catalyst 5.9.
I had a good chuckle posting that. I've been a Catalyst user/fan for around 2 years now and appreciate your work on it. I saw that your slides on KitiWiki used Moose. Exciting that Catalyst will be using it itself. Now I have a fun excuse to play with Moose for real. Bread::Board looks interesting as well, I hadn't seen it at all.
Here's a good example of a Catalyst plugin, so you can see how straightforward it is: http://search.cpan.org/~jrockway/Catalyst-Plugin-Unicode-0.8...