This topic always piques my interest, particularly becuase everyone encounters it at some time in their CGI life. The question is, really: what is the user trying to do during this particular request? One hint is the request type itself: GET, POST, HEAD, etc. Here's what I do (similar to your method, I think):
1. post to the same page with a unique variable, like go=1
2. check for go=1, then do form processing

You could also post to a different CGI or a different subroutine in the same CGI, and simply posting to that CGI or sub indicates that form processing is to take place.

I really enjoy the speed and freedom of mod_perl for this reason... you could probably implement a pretty complex state machine to know at what point a user is in a particular process (logging on, posting a message, etc.).

"This process seems ineficiant and error prone."

I guess this depends on the programmer, right?

If anyone knows more about state machines (links, info, whatever) please post!