I have not hired a Perl consultant specifically, but I do have experience with consultants in general. The most basic step is to first ask exactly the same questions you would ask a potential new-hire. After all, you are expecting the same performance, just on a short-term basis. If you were hiring a business analyst or auditor for your department, the questions would be different, but you are essentially hiring short-term, part-time labor.

The other questions of importance for consultants:

1. Have you done this kind of thing before? Can I see a sample of that work?
2. How long do you think it will take you to do this?
3. Do you have any other projects running simultaneously, or will you be focussing all of your efforts on this project?
4. Project or hourly pay?

There may be other questions that your best judgment will suggest, but I have found these questions particularly helpful and revealing. (Also consider asking him/her to evaluate a short section of code, and see if s/he gives the answers you want)