The following provides a parallel version for the slurp routine. I'm not sure why or where to look, running MCE via cmpthese reports inaccurately with MCE being 300x faster which is wrong. So, I needed to benchmark another way.

Regarding MCE, workers receive the next chunk and tally using a local hash. Then, update the shared hash.

use strict;
use warnings;

use MCE;
use MCE::Shared;
use String::Random 'random_regex';
use Time::HiRes 'time';

my $fn  = 'dna.txt';
my $POS = 10;

my $shrcount = MCE::Shared->hash();
my $mce;

unless ( -e $fn ) {
    open my $fh, '>', $fn;
    print $fh random_regex( '[ACTG]{42}' ), "\n"
        for 1 .. 1e6;
}

sub slurp {
    open my $fh, '<', $fn;
    my $s = do { local $/ = undef; <$fh> };
    my $count;
    $count-> { substr $s, $POS - 1 + 43 * $_, 1 }++
        for 0 .. length( $s ) / 43 - 1;

    return $count
}

sub mce {
    unless ( defined $mce ) {
      $mce = MCE->new(
        max_workers => 4,
        chunk_size  => '300k',
        use_slurpio => 1,
        user_func   => sub {
          my ( $mce, $slurp_ref, $chunk_id ) = @_;
          my ( $count, @todo );

          $count-> { substr ${ $slurp_ref }, $POS - 1 + 43 * $_, 1 }++
            for 0 .. length( ${ $slurp_ref } ) / 43 - 1;

          # Each key involves one IPC trip to the shared-manager.
          #
          # $shrcount->incrby( $_, $count->{$_} )
          #   for ( keys %{ $count } );

          # The following is faster for smaller chunk size.
          # Basically, send multiple commands at once.
          #
          push @todo, [ "incrby", $_, $count->{$_} ]
            for ( keys %{ $count } );

          $shrcount->pipeline( @todo );
        }
      )->spawn();
    }

    $shrcount->clear();
    $mce->process($fn);

    return $shrcount->export();
}

for (qw/ slurp mce /) {
    no strict 'refs';
    my $start = time();
    my $func  = "main::$_";
    $func->() for 1 .. 3;

    printf "%5s: %0.03f secs.\n", $_, time() - $start;
}

__END__

slurp: 0.487 secs.
  mce: 0.149 secs.
[download]