A base class providing a solid implementation of a controllable plugin pipeline. Well-tested as it is an integral part of the popular POE::Component::IRC suite (amongst others) via POE::Component::Syndicator.
The ability to easily control pipeline order (via methods) and event lifetime (via return values) are useful features that often lack in other plugin systems. Plugins are passed references to arguments, and therefore have the ability to alter event arguments before they are handed off to the next plugin in the pipeline.
Worth a serious look if implementing any kind of pluggable class -- POE users should probably also look at POE::Component::Syndicator, a subclass of Object::Pluggable.