Going through inccode.t, I was able to get it working by returning an in-memory filehandle pointed to the string "1":

use PerlIO::Scalar;  # to avoid recursion when creating $fh

sub require_local {
  # do the actual work, per the source I posted earlier
  # ...

  open my $fh, '<', \'1';
  return $fh;
}
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Thanks for finding those tests and pointing me at them!