Sometimes, I like to know "really" what the overhead is for MCE. So, here is something to measure the chunking nature of MCE. Simply comment out user_begin, user_end, and the process routine. That's it.

#!/usr/bin/env perl

use strict;
use warnings;
use MCE;
use Time::HiRes 'time';

die "usage: $0 infile1.txt\n" unless @ARGV;

my $OUT_FH; # output file-handle used by workers

# Spawn worker pool.
my $mce = MCE->new(
    max_workers => MCE::Util::get_ncpu(),
    chunk_size  => '64K',
    init_relay  => 0, # specifying init_relay loads MCE::Relay
    use_slurpio => 1, # enable slurpio
  # user_begin  => sub {
  #     # worker begin routine per each file to be processed
  #     my ($outfile) = @{ MCE->user_args() };
  #     open $OUT_FH, '>>', $outfile;
  # },
  # user_end => sub {
  #     # worker end routine per each file to be processed
  #     close $OUT_FH if defined $OUT_FH;
  # },
    user_func => sub {
        # worker chunk routine
        my ($mce, $chunk_ref, $chunk_id) = @_;
  #     process_chunk($chunk_ref);
    }
)->spawn;

my $start = time;
$mce->process($ARGV[0]);
printf "%0.3f seconds\n", time - $start;
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I have a big file which is 767 MB. The overhead is a fraction of a second.

$ ls -lh big.txt 
-rw-r--r-- 1 mario mario 767M Oct  5 10:07 big.txt

$ perl demo.pl big.txt 
0.154 seconds
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Edit: That was from OS level cache as I had read the file from prior testing.