Note: I'm (as usual) off-topic...

I really like it when languages allow me to be lispy -- even though I really dislike being lispy in lisp! Lots of insignificant parenthesis -- Go figure! I'd say this ability of Perl (to allow funky functional constructs without the painful restrictions of pure-functional languages) is 70% of my love of the language. (CPAN and the data structure support fill the other 30%).

I don't know how many times I've wanted to do something with lamba functions and closures in C, and then I kick myself -- doh -- you can't do that! Java does a little better with the anonymous inner classes, but the syntax is horribly cludgy so I can't give it any browny points. I have this one build engine I'm rather proud of that heavily abuses map and anonymous subs. Why? Well, to keep people from messing with my build system -- err, no, because it's insanely powerful. It is, however, amazing how many self-proclaimed OO Gods can't grok functional code. I like it!

Back on topic: "Left for an exercise for the reader" is bad form, IMHO, in official documentation or a reference manual. It's fine for a student textbook, but it annoys me to no end when a writer thinks they are clever and then won't explain why they think they are clever. Hey, I'm stupid...sometimes I can't figure out the darn exercises! Teach me, or better, give hints.