duplicate the client's requests with mod_proxy (or mod_perl)?

When apache mod_proxy pass the client's request to an original server, I want it
always duplicate the request to another webserver.

For example, the request is following: client -> mod_proxy -> original-server
What I want is:

client -> proxy -> original-server
                     |-> another webserver

The request should always be duplicated, rather than the round robin.

Any idea for doing that?
(I know some about mod_perl API, could write the handler).

Thanks.