I read through the source for Win32::NetAdmin, and it uses the Win32 API call "NetUserGetInfo". According to Microsoft's documentation, "The NetUserEnum and NetUserGetInfo functions return a NULL pointer to maintain password security."

So, the $password is not set with a valid password. Have you tried printing out the value to see if it's correct? Maybe there's junk data in there, or maybe something else in your program is setting it by accident.