You have it backwards. PerlMonks *adds* an unclosed <p> tag at the start of the body of the topmost post. Mind you, there's nothing wrong with unclosed P elements in of itself, but it's messing up the formatting here.

PerlMonks also adds an *unopened* </p> tag at the end of the same post. It's obvious that someone intended to wrap the entire post body in a P element, but that's not possible. P tags can't contain other block elements (such as P, DIV, TABLE, etc), but post bodies can.