I don't know if I am understanding completely, but it sounds like you need to provide a username and password for a site? If that is the case, LWP::UserAgent has the authorization_basic method that can be used something like this (directly from the lwpcook manpage):
Documents protected by basic authorization can easily be accessed like this:
$ua = LWP::UserAgent->new;
$req = HTTP::Request->new(GET => 'http://www.asite.com/');
The other alternative is to provide a subclass of LWP::UserAgent that overrides the get_basic_credentials() method. Study the lwp-request program for an example of this.
Hope that helps.