For a site like this one that caters to programmers, I find it an excellent choice to just give them all the expressiveness they want by letting them use what they already know: HTML.
What's HTML got to do with programmers? If I expect one markup language Perl programmers would know, it would be POD, not HTML. Besides, the language accepted on perlmonks isn't HTML. It has markup that HTML doesn't have (for links for instance), lots of HTML markup isn't allowed (a span element is allowed, but not a style attribute, which makes span not very useful, and there are other elements that aren't allowed), and some elements get a totally different meaning (code).
I don't think anyone knows the language being used on Perlmonks prior to arriving here.
Abigail