There's one question that has been my favourite for a long, long time. It's not Perl specific. I have asked the question, and I've been asked the question. And the answer itself isn't even important. What is important is how well they can defend their case. The question:

What don't you like about XXX, and how would you change it?

From the given answers, you should be able to deduce how well they know the language/OS/DB/application you are asking about, and how much experience they have.