test_read_parallel.pl

use strict;
use warnings;

my $processes  = $ARGV[0] || 1;
my $block_size = 64 * 1024 * 1024;

my $file = 'very_big_file';
my $fsize = -s $file or die 'no file';
my @offsets;
my @sizes;
my @pids;
for my $part (0 .. $processes - 1) {
    $offsets[$part] = int($fsize / $processes * $part);
    $sizes[$part] = int($fsize / $processes * ($part + 1)) - $offsets[$part];
    if (not($pids[$part] = fork)) {
        open my $fi, '<', $file or die;
        my $read = 0;
        for (my $pos = 0; $pos < $sizes[$part]; $pos += $read) {
            my $rs = $pos + $block_size > $sizes[$part] ? $sizes[$part] - $pos : $block_size;
            $read = read($fi, my $buff, $block_size);
        }
        exit;
    }

}

for my $pid (@pids) {
    waitpid($pid, 0);
}