See also URL Tomfoolery; in particular, chromatic's reply (which blames nate (which, to my knowledge, is correct)).

I think this was the Everything-engine way of knowing when a form was submitted, as opposed to just displaying the form. This allowed for a relatively easy way to determine if a setting (such as a checkbox) should be cleared because the user set it that way (such as unchecked the checkbox) or the value is clear because the page was generated without the form being submitted.

It looks like E2 still uses that in a lot of places, but PerlMonks appears to use a different mechanism for probably the most common case (checkboxes, as seen in checkvar).