I would not be that interested if somone would be relatively new to the language, as to see how fast someone would be able to learn it.
Not knowing the exact level needed reading your question, i'd ask about the way a problem would be solved (look at scope does the person thinks in), i'd ask about known and used coding patterns. (is the person a developer or does he / she only type the code that's designed by someone else).
If you really want to know if the person is a merlyn like guru, ask about perl-internals and his OO-perl capabilities as well.
Next, i'd like to see if the person fits in the team you're working in.
I only wonder if this can be done in a 5 minutes screening...