I don't see how the second alternative of the || could ever be false (i.e. the coverage would never be 100%).

It can't, but just like with xdg's note about my $value = $some_other_value || undef;, Devel::Cover is also smart enough to realize that DEFAULT_FILENAME is a constant and removes the truth table entries of it being false, so that you can get to 100% coverage for that line.