You'd think so, but you'd be wrong -- HTTP basic authentication is normally a two pass process -- once for the client to find it needs authentication, and a second request when it actually sends the authentication. In HTTP, a server might have multiple realms associated with it, and the client has no way of knowing which realm it needs to authenticate under without the first failed connection.

See F5 for a work around if you want it to only do one pass. (it also shows directly messing with $soap->transport(), which gives you access to the SOAP::Transport object.)