Since I don't see this term elsewhere in a PerlMonks search and it doesn't appear in the index of any of my dozen perl books, could someone provide a clear explanation and description of 'mixins'? What they are, what they're intended to solve and so on?

Basically a mixin is a partial-class that makes no sense by itself, but can be "mixed in" to another class to supply additional functionality.

The classic example is adding borders to different kinds of windows in a GUI framework. You have your normal GUI widgets and have a border mixin that you use to create widget classes that draw their borders. You add the border drawing behaviour to the classes with the mixin.

So rather than specialising behaviour in a subclass you add additional behaviour with a mixin.

DIfferent OO languages support mixins in different ways. Some like Perl and C++ have no explicit support and you implement them with other features like multiple inheritance or symbol table fiddling. Other languages like Ruby and Flavors have explicit support for mixin classes.

The "mixin" terms comes from the Flavors langauge - one of the earliest object oriented languages that ran on Lisp Machines. The name came from the (in)famous ice cream metaphor used by the language (the base class was called 'Vanilla') due to an popular ice cream store used by MIT students - Steve's Ice Cream.

At Steve's you ordered a code or dish and specified the base flavour and any mixin flavours. You then got a scoop of the base with spoonfuls of mixin flavours on top.