May I suggest using Win32::AdminMisc? It should be installed if you've got the activeperl distribution, and contains the popular and delightful UserChangePassword( $Domain , $User, $OldPassword, $NewPassword ).

Note that this function returns errors through $^E, not $!. At least $! was consistently undef'd when UserChangePassword failed. Also note that your passwords are still restricted to the domain's policies; it's quite feasable that you lock an account out after x tries!