Waving the magic wand and assuming that such a spec were in place, we would revisit my "issue" as posted, and the solution would be something like: print "Client:popup(text)\n\n";

Or you could avoid hand waving and just do

print "Content-type: text/html\n\n";
print "<script>alert(text)</script>";
[download]

Which does the exact same thing your "waving the magic wand .. spec" would do. It pops up a dialog box. What more do you want? If you want to submit form data without clearing the page you simply (relatively speaking) point the form at a different frame, even a hidden iframe and do your communication that way. Theres even a framework or two that will do this for you.