There are actually a bunch of similar tricks you can do in apache to persuade it to always run a particular script for any requests, perhaps the most sensible of which is to use mod_rewrite to rewrite everything to your script (and thus log everything as 200s not 404s).

However if you really want to tell it to always run that script, you should also be able to do so in an .htaccess file this way:

Action my-mirror /mirror/mirror.cgi
SetHandler my-mirror
[download]

...and have another .htaccess file in /mirror/ saying:

SetHandler cgi-script
[download]

...or mod_perl as appropriate. There may be a shorter way, but I'm not in a position to test it right now.

(Edited to fix (english) grammar. Oops)

(Edited to remove inintentionally humourous english link)