use Win32::API qw( );

my $register_client2 = Win32::API->new(
   $S{DLL_path}, 'RegisterClient2', 'PP', 'I',
);

sub register_client2 {
   my $ip = $_[1];
   my $id = pack('l', 0);  # Allocate space
   my $rv = $register_client2->Call($id, $ip);
   $_[0] = unpack('l', $id);
   return $rv;
}

register_client2(my $id, $ip)
   or die;

##</code><code>##

sub register_client2 {
   my ($ip) = @_;
   my $id = pack('l', 0);  # Allocate space
   $register_client2->Call($id, $ip)
      or return ();
   return unpack('l', $id);
}

my ($id) = register_client2($ip)
   or die;