And to make sure, it's clear, $mypage in turnstep's example would be the URL for your form. In other words, if the URL for your form was:

http://www.mydomain.com/pages/myform.html
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you would want to set $mypage equal to that. Also, to make sure it exits gracefully if someone just goes directly to the script, I would put the following early on in it:

$errorURL = 'http://www.mydomain.com/mypage.html';
if (!defined $ENV{HTTP_REFERER}) {
    print "Location: $errorURL\n\n";
    exit;
}
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This will send them back to $errorURL if they try to go to the script directly rather than giving a possible 500 error.