I didn't mean to say there is a problem with CGI::Session directly. I said I had encountered a problem (a strange phenomenon that happens randomly) when using CGI::Session.
With a good night's sleep, I was able to run more tests and came out with new hypotheses about how the bug was triggered. But I need to confirm these hypotheses with real users. So I've taken your advice - thanks :) - to do logging at suspicious points in my code.
I was clueless yesterday because I was looking at the code and could not figure out any parts that could trigger that bug, so I wouldn't know what to log and where.