Actually, this misses one small but important point. The code contains:

recurse_dir  ($_)  if -d;
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This runs into problems when you see the directories "." and "..", so you need to do something more like:

while (readdir D) {
    if (-d) {
        recurse_dir($_) unless ($_ eq '.' or $_ eq '..')
        }
    else {
        process_file($_) if -f
        }
    }
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