By POSTing to snake.cgi with an arbitrary value in the cookie. There are some other values your server or various other scripts set in that cookie, too, but I didn't bother checking what they did; I just copied and pasted.
i know others have said that people could fake their scores, but i cant see how they could do it with mine.
Reproduce: get onto the high score list (or call formfields() manually), then use:
... in your browser's address bar. Enter a name and submit.