I always feel bad putting a candidate on the spot with "tricky" questions, but on the other hand I want to give exceptional candidates a chance to stand out. I have a favorite question which lets me sneak some advanced stuff in while not putting unfair pressure on non-advanced candidates:

Take a look at the following short snippet of code. There are some problems with it. Can you point out the problems and explain them?

I include syntax errors, programming logic errors, some really bad style elements and a couple of debatable style problems. Some of them are really obvious, but others are very subtle. It gives the good communicators a chance to show how well they can explain things, and the knowledgeable Perl programmers a chance to show off. At the same time it doesn't put anyone on the defensive, because there are a few easy-to-spot errors.

Another open-ended, conversation-starting question I like: write (or describe how you would write) code that performs some specified task, without using particular perl operators or functions. I asked one question like this during our last round of interviews -- manipulate a given array without using for/foreach/while/until -- and was pleasantly surprised to get several different correct answers. When I wrote the question, the keyword I was fishing for was "map", because a lot of beginners seem intimidated by it or completely unaware of it.

Joe