Your results are inconclusive, as -d _ without the parentheses would be enough. Maybe if you pit the total of -X $something against the total of -X _ plus -X(_) plus -X (_) etc you would have a better idea.

Anyway, the point is moot because (IMHO!!) this:

  if( -x $filename and -d _ )
[download]

is better, cleaner, more concise, and clearer, than

  if( -x $filename and -d $filename )
[download]

because I am used to Perl and I know that anything that has _ in it means that thing we were just talking about, i.e., the Topic.