I don't believe that I have ever come across anything that explained...
You just linked to it. A general purpose dictionary should answer many questions.
[T]he meaning of uncommon words should be explained the first time they are used
That is a good goal, and all tutorial writers should follow it appropriately. I don't think it's the right goal for reference pages, though. How do you define "first"? How do you choose exactly which examples to highlight?
I'm not convinced that there are a lot of people learning a computer language without some practical goal in mind. Perhaps a better question is, "How can we teach Perl in the appropriate context of solving problems?"
Don't let me stand in the way of doing something useful, though. :)