After the pre-screening interview, I would suggest to make them write a semi-trivial script in real time, that is, in the interview situation, on a prepared PC with the standard tools (editors etc.), for a standard problem situation that comes up in your domain (e.g., web script, DBI connector, traverse directories, etc.).

Look at *how* they approach the problem. Do they re-invent the wheel? Do they consult the online documentation? How do they use the tools provided (editor, debugger, ...). Which questions do they ask you about the problem? Do they make an "analysis" of your requirements? How do they react to this stressful situation? And so on.

Since this procedure is pretty hard-core (emotion- and time-intensive), you will probably want to make sure to limit the number of candidates invited, to make sure you have adequate time for this procedure (e.g., 30-60 minutes).

Just my 2 cent.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com