One way to do it that seems to work on my limited testing is something like this:

#!perl

## Do the work here, process the form, etc. then:

print "Location: http://www.foobar.com/StaticWithFormStuff.html\n\n";
[download]

Refreshing brings up the "new" page, not the script that directed them there.

A second way is to set the page as "expired" - some browsers will then complain if you try and refresh it.

A third way is to keep track of the page via a unique URL or a hidden tag and store this info on the server somewhere. If the unique tag comes up twice, you've got a "repeat offender." Lots of overhead for the server, but the most reliable solution, as you are relying on your server to do the work, and not the unknown behavior of the user's browser.