Lately I've been playing Lacuna Expanse (www.lacunaexpanse.com), a free new MMOG whose back end is written entirely in Perl. I've been pretty impressed with it so far. What's really impressive is that the company behind it is well known for supporting the Perl community and given that I've been so heavily involved in Perl marketing, I'm quite pleased to see a project this significant, written with Perl as the back end.
What's even more interesting, if you're a developer, is that they have an API you can develop for. Plus, if you feel creative, you can download their mission editor and submit missions that they might include in the game. So they encourage a lot of public participation in growing their game (not many games can claim this), but they've also released a lot of their code to the CPAN so developers can take advantage of new tools and techniques they've developed.
Give it a try! It's free, it's fun, and it's a great tool to market the technology I specialize in :) (I'm the "Ovidian Order" in that game).