Don't use the term "Thread" - instead Google for "forum" or "Bulletin board".
Here's a good summary for php ones:
http://en.wikipedia.org/wiki/Comparison_of_Internet_forum_software_%28PHP%29
And here's a list for a variety of languages including Perl:
http://en.wikipedia.org/wiki/Comparison_of_internet_forum_software_%28other%29
Also, you can have pre-canned solutions - a new favorite among heavy hitters seems to be DISQUS - at least a couple of big blogs I follow recently switched to it.
One more option is of course to leverage Stack Exchange - if you can convince enough people to organize it, you can get enough mass to create a StackExhange based site on Area51 (in case you aren't aware, StackExchange is the infrastructure/software behind StackOverflow)
Now, if you prefer to code on on your own, it is also fairly doable in Perl with some database back-end... you basically set up the tables for users, posts, and post relationships, and a basic thread renderer is just printing the tree of posts based on those relationships.