Albeit this is not directly perl related, both the tamperdata and firebug extensions for the firefox browser offer a fairly straight forward "HTTP inspector/sniffer" interface.

However, you might find it more beneficial to your perl knowledge to use the LWP Post command, or even roll your own proxy along the lines of HTTP::Recorder which can be used to create automated test scripts. ( http://www.perl.com/pub/a/2004/06/04/recorder.html )