I have a lot of fun coding Perl to just about any application that allows for a Perl interface. My IRC client (XChat) and my AIM client (Gaim) both allow for extensions in Perl, which is an absolute riot, and allows for some event-driven programming, which is something I don't get much opportunity to do in Perl outside of Tk.

Extending applications in Perl seems small until you start to think about what outside applications, etc., you can tie into. Databases, web servers, whatever. I don't know if any of this is earth-moving work, but it's a hell of a lot of fun.