As others have already suggested, you can use log4perl.

The next thing, which is probably more important, is what to log. Base on my experiences, those are the three most important types of information that will definitly help you to debug your code, and to understand potential problems:

• Entering and exiting a sub;
• The pameters passed to a sub call and the parameters a parameter received (log both, don't assume that what you passed in is what being received);
• Information help you determine which logical branch is actually executed.

For multi-threading programs, have a thread id attached to each thread, so logs are lucid.