CPAN.pm can do anything that the user running it can do. A "test.t" file could erase all your files, or replace your binaries with trojans. (Actually, even installing as root could create trojans.) Not running CPAN.pm as root is a very good thing.

Given your situation, you couldn't have stomped on system files, most likely. I'd try creating a new user and seeing if that new user can run tcsh. If so, then it's reasonable to conclude that something in the CPAN install might have caused some local problem for your original username -- something in ~/bin maybe? That said, I have a hard time understanding what it could have done. You might see if there are any outstanding processes -- I find that hitting CTRL-C in the middle of a test run often leaves perl running in the background chewing up processor cycles. Again -- not sure what impact that would have, but it's worth checking. Have you rebooted the machine?