To customize the response to the browser, you will need to capture the output from the cgi script. For cgi scripts at another machine, your Apache handler will in affect act as a forward proxy to the request: it will send the request with LWP::UserAgent and capture the output with LWP::Response.

There is no difficulty if you know the Apache API. Just ensure sure that all incoming and outgoing headers are copied, especially the HTTP headers that may appear twice, like Cookie -- you cannot use functions like $r->headers_in that return hash structures, thus suppressing the second Cookie header.