use MCE::Loop max_workers => 4, chunk_size => 'auto';

   ## processing a file directly (mce_loop_f) takes 0m11.7s

   mce_loop_f {
      my ($mce, $chunk_ref, $chunk_id) = @_;
      chomp @{ $chunk_ref };

      my @o; for (@{ $chunk_ref }) {
         push @o, $_;
      }
      MCE->say(@o);

   } '/path/to/four_million_rows.txt';