Have you checked out the documentation?
http://search.cpan.org/author/GBARR/libnet-1.12/Net/SMTP.pm
indicates that there is an Auth method that takes user,pass something like;

$smtp = Net::SMTP->new('mailhost', 
      Hello => 'my.mail.domain' 
      Timeout => 30, 
      Debug   => 1, 
      Auth => 'username,password' 
      );