How do I create a lighttpd proxy rule to redirect on differant ports based on url param

Question

I currently have a proxy.server rule in lighttpd.conf that forwards all requests of routemsg.pl to port 1530:

$HTTP["url"] =~ "/routemsg.pl" { proxy.server = ( "" => ( ( "host" => "127.0.0.1", "port" => 1528) ) ) }

How can I change the rule to allow the requester to pass a port param in the url and that param be then used as the port to proxy the request to?

For example: A request of: http://www.myip.com/routemsg.pl?p=1531 would go to 127.0.0.1 on port 1531.