This is a very difficult question. About the best you could hope for is to weed out the totally inept, and hope whoever's left is reasonably competent.

The problem (IMNSHO) is that there hasn't yet been developed a consistent measure of code quality. Is it better for someone to crank out lots of workable code, or to write a few terse lines that get the job done? We might be tempted to choose the terse programmer, but perhaps the verbose one included code to check for inappropriate input; or (s)he wrote code that would be much easier to understand, debug, and maintain. What objective test could you use to decide a question such as this?

As I see it, fluency in Perl is only part of the question. Another part is the ability to develop appropriate, efficient algorithms. Without this, the fluency will be wasted. They will express their unclear thinking in fine Perl idiom.