What I'd really like to see is not so much a MUD written in Perl (though that's fine), but instead, one that uses Perl as the in-game programming language. (Instead of the lisp-like MUSH code, or c-like stuff in a MOO or an lp/diku.) I spent a lot of time on roleplaying TinyMUSHes in a previous life, and it would be awfully nice to be able to implement the complex systems that people implement (like entire RPG rulesets) in perl, rather than MUSHcode. That stuff is ugly!
I considered once doing a mod-perl like extention to MUSH, but that was shortly before I pretty much stopped playing. It would be an interesting (if huge) project.