This isn't exactly fair if they have mostly commercial work that they cannot share. I know this is true of me -- I want to code in my spare time, but 50 hrs a week of staring at a monitor is usually enough for me.
I think the best is to measure analytical skills, ask them to walk you through a previous design, or so on .. or maybe ask them to explain one of your Perl programs.
But don't get TOO technical. I had a few interviews from PhD types who thought they were Gods gift to computer science (giving a bunch of problems they thought were clever, but really weren't -- and then made up arbitrary restrictions to limit my ability to solve them through 'normal' means). They rubbed me the wrong way, were full of themselves, and I wouldn't have wanted to work for those sort of elistists.
So it comes down to ... (1) can they work and play nice with others, and (2) can they think on their feat and do they have a general grasp on good design, and (3) I think it's perfectly fine to ask if they understand closures and lexicals. You see, this is what distinguishes a basic hack-and-slash 'scripter' from someone who is into the stuff from a programming perspective. Maybe. Just don't become that PhD jerk who thinks he is smarter than the interviewee. Intimidation does nothing for anyone, and will problably just make your subject feel uncomfortable.