See run your own perlmonks! for a long explanation of my views, and that of many other people, on the subject.
Short summary: Perlmonks is based on an old (0.9, IIRC) version of The Everything Engine, as is noted at the bottom of every page (didn't look very hard, did you?). Everything is Open Source ("under the same terms as perl itself", IIRC). However, there's been quite a bit of divergent evolution since then, and the exact code that Perl Monks runs on is not available, execept to the gods and members of pmdev. (In case you were wondering, since we distribute only output, not binaries, most of the GPL does not apply. In any case, we could go with the Artistic License, and be pretty sure that we're not breaking with the "artistic vision of the original author", since the orignal authors are mostly gods, and if they objected they could easily do something about it. It's quite legal, I just don't agree with it.)
Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).