You could use Courier::Filter::Module::Header to filter mail into a spam bin based on key words in the subject, but so much spam has obfusicated in so many ways that a simple filter based on key words will not be very robust.

A better option would be to use Courier::Filter::Module::SpamAssassin.

However I suspect that neither of these options are a good point to be starting out in Perl unless you have a fair amount of general programming experience and are a fast learner. Good luck with it in any case and come back if you run into trouble.