Someone's already mentioned Log::Log4perl, but let me just add another vote. I use it in Production code, and if Something Goes Bad, instead of hacking code on a Production server (never a good idea), I can just turn up the debug level from WARN to DEBUG and watch a few web requests go by in the log file.

There are also lots of options .. for example, instead of sending messages to a log file, they can be E-Mailed, sent out a serial port for external logging, etc., etc.

Highly recommended.