A good Perl programmer knows when and how to use the Perl modules that come with the standard installation, always uses strict and warnings (and where applicable, tainting), and sections his code into manageable blocks with defined inputs and outputs. This should be the minimum for anyone you are looking to hire. Of course, he'll also need to be familiar with Perl syntax (specifically, nested structures and references and so on), with all the material in perlfunc and perlvar, and with everything you'd learn in a standard Data Structures course.

Outside of that, you'll probably also want someone who works in a standard coding / commenting format, and who knows at least one other programming language (C/C++ or PHP). But this isn't absolutely essential.