I personally like "What's your favorite module and why?" Tells you alot about someone's programming experience and their programming preferences, and it gives the interviewee a chance to be clever if they want. "CGI.pm", and "DBI.pm" are solid, straightforward answers... "Quantum Superpositions", however, tells a completely different story. :-D
Of course if they can't answer it at all that tells you something, too. The idea is not to rigidly test the applicants (which can be problematic for a variety of reasons) but rather to elicit the sorts of responses necessary to find out if they have skills and competencies to do your work.