Here's the general direction I'd take with respect to Perl knowledge (vs. a lot of other things I'd want to consider when hiring):

A) Tell me what sorts of things you have used Perl for.

B) Have you used the object-oriented side of Perl a lot? If so, tell me about a module you have written. What were your considerations in creating objects and methods?

C) How familiar are you with the resources at CPAN? [Here, if I had a particular knowledge area (such as, say, bioinformatics) needed for the job, I'd ask if you were familiar with modules for that particular application area. Even if there's no current need, I'd want to know if you've used DBI and related modules to work with a database.]

D) Have you ever found that someone else's module came close, but not quite close enough, to doing what you wanted? What did you do then? [Looking to see whether the impulses towards rewriting vs subclassing are in proper balance.]

E) Have you had occasion to either extend Perl with functions written in another language?  Have you ever had occasion to embed Perl into another application?

F) What do you know about Perl compilers? Have you used them? What problems did you encounter, and how did you resolve them?

G) When you are developing a Perl program, how do you go about it? Do you have a favorite editor or IDE? (Have you even _tried_ an IDE?) How do you debug your Perl program? Tell me about a recent bug you had, how you went about uncovering it and what you think caused it.

H) Are there any other Perl-oriented products that you have experience with? [e.g. extending VIM with Perl? Apache mod_perl? Mason?]

I) Can you contrast the features and qualities of Perl with some other programming or scripting language? What things do you consider Perl to be especially well-suited for? [And are there things for which you don't consider perl to be well-suited?]

J) What can you tell me about Perl6? Parrot? [Just to see if you're keeping up-to-date.]

I would expect through the discussions arising from these questions to get a better handle on how well you know various details of Perl, something about your style of solving programming problems, and perhaps how well you'd be able to interact or mentor other Perl programmers.