Ловушки Perl

mysql> show create table movie_titles\G
*************************** 1. row ***************************
       Table: movie_titles
Create Table: CREATE TABLE `movie_titles` (
  `movie_hash` bigint(20) unsigned NOT NULL,
  `title` varchar(255) NOT NULL,
  `count` int(10) unsigned NOT NULL,
  `min_year` int(10) unsigned NOT NULL,
  `max_year` int(10) unsigned NOT NULL,
  `min_budget` int(10) unsigned NOT NULL,
  `max_budget` int(10) unsigned NOT NULL,
  `title_lang_id` varchar(2) NOT NULL DEFAULT '',
  PRIMARY KEY (`movie_hash`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select movie_hash, title from movie_titles order by rand() limit 10;
+----------------------+-----------------------------------+
| movie_hash           | title                             |
+----------------------+-----------------------------------+
| 14466895222078503050 | Короткие встречи                  |
| 15971437470478867358 | The Intouchables                  |
| 12235011833542755999 | Карнавальная ночь                 |
|  6981806602143484496 | Ben-Hur                           |
|  5919830798737205613 | Зеркало                           |
|  9358350811894903428 | Ikiru                             |
| 15397825164883494235 | City Lights                       |
|  1081902961608193125 | Я шагаю по Москве                 |
|  6154127428560052859 | The Kid                           |
| 16809407873175270375 | Безымянная звезда                 |
+----------------------+-----------------------------------+
10 rows in set (0.01 sec)

mysql> select movie_hash % 4, count(*) from movie_titles group by 1;
+----------------+----------+
| movie_hash % 4 | count(*) |
+----------------+----------+
|              0 |  1217109 |
|              1 |  1217068 |
|              2 |  1217333 |
|              3 |  1216358 |
+----------------+----------+
4 rows in set (2.78 sec)

#!/usr/bin/perl

use strict;
use warnings;

use DBI;
use Parallel::ForkManager;
use Log::Log4perl qw(:easy);
 
use utf8;
use open ':std' => ':utf8';

Log::Log4perl->easy_init( 
    { 
        level   => $INFO,
        file    => ">>process.log",
        layout => "[%P] %c %d: %m%n",
    } 
);


INFO("START");
my $WORKERS = 4;
my $pm = Parallel::ForkManager->new($WORKERS);

for my $num (0 .. $WORKERS - 1) {
    my $pid = $pm->start and next;
    process_part($num, $WORKERS);
    $pm->finish;
}
$pm->wait_all_children;
INFO("FINISH");
exit(0);

sub process_part
{
    my ($num, $total) = @_;

    INFO("$num/$total - start");

    my $dbh = get_dbh();

    my $data_to_process = $dbh->selectall_arrayref(
        "select movie_hash, title from movie_titles 
        where movie_hash % ? = ?", 
        {},
        $total, $num
    );
    INFO("to process: ".scalar(@$data_to_process));
    for my $record (@$data_to_process) {
        do_some_heavy_processing($record);
    } 
    INFO("$num/$total - finish");
}

sub get_dbh
{
    # ...
    return $dbh;
}

sub do_some_heavy_processing
{
    # ...
}