While I would argue that the inheritance tree of File::Spec is not an internal detail because it is documented, you are right: File::Spec is intended for filename handling, and checking the style of filenames in use does not reliably identify *nix systems because several Perl ports translate Unix-style filenames to the native format.

For the release that I am calling "alpha-5" for short, I am using this code:

BEGIN {
  use constant ();

  my $have_valid_inodes = 0;

  # We accept DEV:INO as valid if two files in the same directory have
+ the
  #  same DEV and different INO values.  We use two modules from this
  #  library for this test and retrieve their actual locations from %I
+NC.
  my @stat_record = stat $INC{'WARC/Record.pm'};
  my @stat_volume = stat $INC{'WARC/Volume.pm'};

  $have_valid_inodes = 1
    if (scalar @stat_record && scalar @stat_volume # both stat calls w
+orked
    && $stat_record[0] == $stat_volume[0]       # both have same DEV
    && $stat_record[1] != $stat_volume[1]);       # different INO valu
+es

  constant->import(HAVE_VALID_INODES => $have_valid_inodes);
}
[download]

What makes this particularly "interesting" is that all of the CPAN smoketest boxes seem to be running various *nix systems.