Let's say that you have a frontend server running mod_ssl, mod_rewrite, and mod_proxy. You want to make sure that your user is using a secure connection for some specific actions, such as login information submission. You don't want to let the user log in unless the request was submitted through a secure port.
Since you have to proxypass the request between the frontend and backend servers, you cannot know where the connection originated. The HTTP headers cannot reliably provide this information.
A possible solution for this problem is to have the mod_perl server listen on two different ports (e.g., 8000 and 8001) and have the mod_rewrite proxy rule in the regular server redirect to port 8000 and the mod_rewrite proxy rule in the SSL virtual host redirect to port 8001. Under the mod_perl server, use $r->connection->port or the environment variable PORT to tell if the connection is secure.