The cron utility runs under a very limited environment. This means that you have to spell things out in a way you would not if you were working interactively at the command line (e.g., /usr/bin/perl rather than just perl).
It's also worth saying that Apple recommends launchd over cron. (I actually use cron myself on Macs, but only because I'm used to it from Linux machines. I keep meaning to switch.)
