One other thing. On MacOS X (and it's predecessor NeXTStep) there is an explicit structure used in application development called a Framework. Search around on http://developer.apple.com for more info if you are hearing about these frameworks in a Macintoshian context..

It's essentially a library of functions evolved a step or two - it can include executable code, headers, resources, documentation, translations of strings, and other such stuff.

It's not directly perl-related, true, but you can use perl to develop Frameworks (see Camelbones), and there are perl-related Frameworks you use from time to time.