You don't say what operating system you are using and what problem you are trying to solve, but the easy solution is to set up a proxy and make your browser use that proxy. There is a convenient proxy package availble with HTTP::Proxy. On the other, harder side, there is the low-level manipulation possible with libnet and libpcap (Net::PCap), both of which have injection and manipulation callbacks, and if you're using a BSD (-based operating system), you might find the port filters by Stephanie Wehner useful.