Its an open bug?

Thanks for that. One of those links led me to XML::Parser and how it deals with the problem. And then I remembered that some of Tassilo von Parseval's modules parse the command line for INC and LIBS args, so I took a look at his Unix::Statgrab Makefile.PL.

Seems to work fine if the top level Makefile.PL does:

our %args = map { split /\s*=\s*/ } @ARGV;
our $LIBS_LOC = $args{ LIBS } || "-lgmp";
our $INC_LOC = $args{ INC };
[download]

and then the subdir Makefile.PL simply has to specify:

WriteMakfile(
  ....
  LIBS => $LIBS_LOC,
  INC => $INC_LOC,
  ....
);
[download]

I haven't yet looked at any caveats with that approach. I'll get to that now.

Thanks again.

Cheers,
Rob