preprocess(): you pass in a list of files, set a flag, and then pass the same list back...seems to me that all you need to do is set the flag...

preprocess() is intended to be used as a filter for the list of files returned by readdir(), so it is expected to return a list, which in turn replaces the original list returned by readdir().

Eliminating the unnecessary assignment, preprocess() could be reduced to:

sub preprocess {
  print "Pre-Processing\n";
  $old_flag = 0;
  @_;
}
[download]

This would probably be a little more efficient, but I'd be inclined to look for an even more efficient solution that would eliminate the need to pass the entire file list around just to set a flag.