I put essentially this very question to colleague on an earlier project, he thought about it a couple of days and came back with way of invoking php from perl and perl from php.

What I remember was (untested):

<? php include("/path/to/script.cgi");  ?>
[download]

with the script written not to duplicate headers, but to inject html, ready for inclusion, into the including page. You could even let the wordpress application handle the authentication and only include the limited access cgi script on the protected page.

If I come across my notes on going the other way, I'll update this thread.

-- Hugh