PerlMonks does use CGI.pm. It's a fork named Everything::CGI.
Development of the PerlMonks engine diverged from Everything a long time ago. I'd estimate about 20 years ago. There has been some porting of features back and forth over the years, but very little. My point is that studying the Everything Engine you download now will only give you the most general gist of how PerlMonks works.
If you like, I can add you to the PmDev group so you can start looking at the code here.
I reckon we are the only monastery ever to have a dungeon stuffed with 16,