"preprocess"
          The value should be a code reference. This code reference is used to
          preprocess the current directory. The name of the currently pro-
          cessed directory is in $File::Find::dir. Your preprocessing function
          is called after "readdir()", but before the loop that calls the
          "wanted()" function. It is called with a list of strings (actually
          file/directory names) and is expected to return a list of strings.
          The code can be used to sort the file/directory names alphabeti-
          cally, numerically, or to filter out directory entries based on
          their name alone. When follow or follow_fast are in effect, "prepro-
          cess" is a no-op.