sub MY::xs_o

[Sisyphus]

Hi,
I'm wanting to insert my own sub MY::xs_o() into a Makefile.PL, but I 
want the sub to be visible only if $Config{make} eq 'nmake'.

So I wrote in the Makefile.PL:
if($Config{make} eq 'nmake') {
  sub MY::xs_o {"text to insert in Makefile"}
}
That worked fine - except that sub MY::xs_o() is visible (and is 
enacted) even if $Config{make} is *not* 'nmake'.

Is there a way I can get the behaviour I'm after ?
Cheers,
Rob