In thinking about this question I thought, "maybe -z returns true for an empty directory -- that might seem like a good idea". Trying it on Windows (Active Perl 5.6.1) it seems that all directories return true for -z! But wait, on Linux (again 5.6.1), all directories return false for -z (presumably because they contain links to . and ..).
Is this behavior documented somewhere? Is it intentional? And, most importantly, is it reasonable?