I also remember that the first web based forums used a Perl script that actually created new HTML files on the web server when someone posted a new topic, and modified the existing one when someone posted a reply.

A bit similar to static web site generators today, only with endless security vulnerabilities.

You might be referring to the venerable wwwboard from Matt's Script Archive, which amazingly still exists: https://www.scriptarchive.com/wwwboard.html

It's been almost 30 years, so my memory is hazy, but that really looks like it.

It still exists but last updated in 2000. It might be fun to make a Docker container with the right dependencies (if that's still possible) to run it.

I believe UBB (Ultimate Bulletin Board) worked this way too.

I do! And still thought it was the case! In fact the first reaction to this txt thing was: well, we already have them in html, why txt?

Remember? I just exported bookmarks as a HTML a few days ago. :o

I was referring to the time when Bookmarks.html was Netscape/Mozilla/Firefox’s actual on-disk storage format, not just its import/export format. Editing that file would change the contents of your Bookmarks menu.