I've been playing with the additions to the regex engine in 5.9.5, mostly as a last minute article on named captures for The Perl Review . A lot of this is really cool stuff, as in "Why wasn't it always this way?!" and "How did I ever live without this?!". Check out perldelta for 5.9.5.
I've now also run into Yves Orton's (demerphq's) Perl 5.10 Advanced Regular Expressions slides, first dated from the 2006 London Perl Workshop. Not only does he show the new features, but he goes into some of the motivations and internals for them.
And, after talking to Randal for a few minutes about this, we realized that a lot of this should be in Learning Perl, and that a lot of the stuff in perlfaq6 might get a lot easier. I'll have to practice these recursive regexes to update those answers.