There are still places where you'd want to use Parse::RecDescent over YAPP, based on what I've looked into the latter, a couple which concern me for a project I'm working on; first, you cannot match code blocks if the outside language is not pure perl, unless you insert the entire perl grammar tree within your recipe. This is not a fun prospect. Secondly, YAPP requires you to work with outside tools to generate .pm files that basically become your parser; this doesn't allow for dynamically changing rules or easily creating new parser sets on the fly.
Mind you, this latter condition is probably a rarity in terms of programmers needs; most programs that use such parsers will have a fixed grammer where the improved reliability of YAPP will play out much better. (Maybe I'll play with that as well and put up a comparible review).
Dr. Michael K. Neylon - firstname.lastname@example.org
"You've left the lens cap of your mind on again, Pinky" - The Brain