See Win32:Getting the Full Name from User Manager.

Update Win32API::Net can be used as well:

use Win32API::Net;

my %info     = ();
my $server   = "wonderland";
my $username = "The Mad Hatter";

UserGetInfo($server, $username, 10, \%info);
print "$username is $info{fullName} \n";
[download]

That code is untested (no access to a Windows machine), but it should work after you correctly set $server and $username. ;-) See the POD for Win32API::Net for more information about what other user info you can obtain.

Update 2 Looks like I forgot about the domain part. I'm not sure that first link will help any.