Well, perhaps it's not exactly a mainframe, but there was an interesting thread a while back called Perl on IBM AS/400?. I worked for a while getting Perl going on that platform, and although I was ultimately successful, it was certainly not as easy as running Perl on, say, Linux or Win32, obviously.
in reply to Perl and MVS
That said, I think you'll find that if you are already familiar with the platform, you'll have a fairly easy time getting Perl running, although you might have relatively sparse support for native system stuff (i.e. weird filesystem quirks, forking might not work quite right, etc) or other general funkiness. ;-) Let us know how it works for you, it sounds like it could be a real adventure!