Then one solution would be to include Spec.pm with your distribution, and have your Makefile.PL install it as Cons::Spec (or whatever namespace the CPAN registrars gave you) in the absence of a detected suitable File::Spec. You can load the code in your modules like this:

BEGIN {
  for (qw(Cons::Spec File::Spec)) {
    eval "use $_"; last unless $@;
  }
  die "Cannot find Cons::Spec or File::Spec" if $@;
}
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A little bit of runtime launch cost can provide a ton of flexibility for your installers. If you want, you can even edit this at compile time into a simple use File::Spec, and avoid the runtime cost.

-- Randal L. Schwartz, Perl hacker