What else should I be looking at to try and debug this problem

Well, cron runs its entries with a very limited environment. So for example, if you've set a custom PERL5LIB environment variable, such as by using local::lib to install modules into your home directory, then those settings that perl gets via the environment won't be present. If that's the problem here, there are a couple of possible solutions: you could set PERL5LIB explicitly in the crontab, you could use lib, write a wrapper script, or, IIRC I once did this with an file in /etc/default, but that might be overkill on a simple "single-user" RPi.