Apparently you use credentials as in $soap->transport->credentials... see LWP::Authen::Ntlm and install Authen::NTLM

See also How to Call a .NET-based Web Service Using the SOAP::Lite Perl Library and Contact Webservices on Microsoft Exchange Server and XML::Compile::FAQ/https://www.ebi.ac.uk/Tools/webservices/tutorials/06_programming/perl/soap/xml-compile-soap/XML::Compile::SOAP/http://perl.overmeer.net/xml-compile/#doc

Also, I hate SOAP :)