• Speed efficiency is moot, since IO will limit speed.
• Memory efficiency is as about as good as you can get.

Bugs should be a bigger concern.

• You're treating text as a regex pattern in s/$topdir/$recreate_to/.

  s/\Q$topdir/$recreate_to/
  [download]

• You're using an unlocalised global (*DIR) in a recursive function. You're not using it in a way that would cause a problem, but why?!

      opendir (local *DIR,$dir)
      || die "unable top open dir $dir $!";
      my @files = grep(/\w+/,readdir(DIR));
      closedir (DIR);
  [download]

  Better:

      opendir (my $dh,$dir)
      || die "unable top open dir $dir $!";
      my @files = grep(/\w+/,readdir($dh));
      closedir ($dh);
  [download]

Update: s/close/closedir/

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