When/whereever you create the SOAP::Lite instance, you need to create it with the keep_alive option set to a true value:

     my $soap = SOAP::Lite->new(
        proxy    => [ $url, keep_alive => 1, ],
...
     );
[download]

I don't know if the stubmaker script creates sensible code. Most likely you will need to monkeypatch the subroutine like you did with get_basic_credentials already.