print File::Spec->rel2abs(
    File::Spec->catdir(
        dirname(__FILE__),
        File::Spec->updir(),
    )
);
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Bah, that is ugly! I'd do it another way.

Did you know that rel2abs takes an optional second parameter, the reference directory? So, a much cleaner way (IMO) to do this is:

print File::Spec->rel2abs(
    File::Spec->updir,
    dirname(__FILE__)
  );
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Even though you probably can just do

print dirname(dirname(File::Spec->rel2abs(__FILE__)));
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You may probably even drop the rel2abs call, in this specific case, as __FILE__ would appear to return an absolute path – even though I don't quite trust that to always be the case.

print dirname(dirname(__FILE__));    # ?? Is this reliable?
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All code snippets, mine and yours, show the same string in my test.