Apache::AuthCookie allows you to intercept a user's first
unauthenticated access to a protected document. The user will be
presented with a custom form where they can enter authentication
credentials. The credentials are posted to the server where AuthCookie
verifies them and returns a session key.

The session key is returned to the user's browser as a cookie. As a
cookie, the browser will pass the session key on every subsequent
accesses. AuthCookie will verify the session key and re-authenticate
the user.

All you have to do is write a custom module that inherits from
AuthCookie.