What you want to do is to add auto-authentication to a particular web site. I'll talk about how you could do this with the next version of HTTP::Proxy, which will be out someday, when I have more time to work on the remaining details.

I must admit I haven't read your code that much, and that the old API is a little behind me now, but:

• You simply have to add a filter that adds the correct WWW-Authenticate: header (you can sniff the connection, for example). You shouldn't receive any 401, since you are authenticated.
• You should only push this filter for request that you need to authenticate:

  proxy->push_headers_filter(
      host    => 'www\\.example\\.com',    # it's a regexp
      request => sub {
          my ( $headers, $message ) = @_;
          $headers->header( Authorization => "Basic cm9verMSZXZhadDpRHRq
  +" );
      }
  );
  [download]

• There is no need for a response filter, if the answer you get is a 200. If you are correctly authenticated by the server, it won't send back a WWW-Authenticate: header.
• If you send the wrong password, you'll get a 401.
• Did you try Basic instead of BASIC?
• A possible application would be a filter that catches WWW-Authenticate: and Authorization: to store/cache authentication information and reuse it for all users.

In your post you say:

the docs for the module are quite basic

I resent that... ;-) In version 0.09, the HTTP/Proxy.pm file is 974 lines long, and there are 303 lines of pod documentation. Please tell me what's missing, so I can improve that in the next version (where 12 new modules will appear).

Warning: The HTTP::Proxy API will change a lot in version 0.10. It should be much easier to write simple filters, and a lot of helper filters and examples will be included. The latest CVS snapshot is always available from http://http-proxy.mongueurs.net/.

For those interested, an example proxy is running most of the time on home.bruhat.net:8080. (It's a proxy: you need to configure your browser to use it.)