an extremely strict check that is supposed to only let particular URLs pass and reject the rest

I think you could make your life much easier if you'd reverse the logic of your script: similiar to:

if ($referrer =~ m!^https?://(forum\.)?robidu\.de/!)
  { print "pass\n"; }
else
  { print "reject\n"; }
[download]

That way you can concentrate on what you want instead of what to avoid.