I would probably look around the code they already wrote in PHPerl a Cish Perl and see how could they be improved. I would teach them the tools or the thinking that was missing when they wrote that way. During the day we would refactor their code.

This would mean they are already familiar with the specific problem the code needs to solve. They also return with some physical evidence of that they did in that day not "only" with what they have in their head.