Hopefully it isn't bad form to simply post links to a separate website, but last year I wrote about some plugin-type work I did with OpenInteract2. They talk about a fairly straightforward method to allow people to create plugins by having a module under a particular package anywhere in @INC, and having each plugin be able to specify enough metadata for an external process to order them as necessary.

• A little bit of Algorithm::Dependency and decomposition
• Using a lot lately: Factory + implementation autodiscovery