foo|field2|field3|item1;item2;item3;item4;itemN|field5|field6|field7
bar|field2|field3|item1;item2;item3;item4;itemN|field5|field6|field7
baz|field2|field3|item1;item2;item3;item4;itemN|field5|field6|field7
...

##</code><code>##

000000000000001|item1|foo|field2|field3|field5|field6|field7
000000000000002|item2|foo|field2|field3|field5|field6|field7
000000000000003|item3|foo|field2|field3|field5|field6|field7
000000000000004|item4|foo|field2|field3|field5|field6|field7
000000000000005|itemN|foo|field2|field3|field5|field6|field7
000000000000006|item1|bar|field2|field3|field5|field6|field7
000000000000007|item2|bar|field2|field3|field5|field6|field7
000000000000008|item3|bar|field2|field3|field5|field6|field7
000000000000009|item4|bar|field2|field3|field5|field6|field7
000000000000010|itemN|bar|field2|field3|field5|field6|field7
000000000000011|item1|baz|field2|field3|field5|field6|field7
000000000000012|item2|baz|field2|field3|field5|field6|field7
000000000000013|item3|baz|field2|field3|field5|field6|field7
000000000000014|item4|baz|field2|field3|field5|field6|field7
000000000000015|itemN|baz|field2|field3|field5|field6|field7
...

##</code><code>##

use strict;
use warnings;

use MCE::Loop;

my $infile  = shift or die "Usage: $0 infile\n";
my $newfile = 'output.dat';

open my $fh_out, '>', $newfile or die "open error $newfile: $!\n";

sub preserve_order {
    my ($fh) = @_;
    my ($order_id, $start_idx, $idx, %tmp) = (1, 1);

    return sub {
        my ($chunk_id, $aref) = @_;
        $tmp{ $chunk_id } = $aref;

        while ( my $aref = delete $tmp{ $order_id } ) {
            foreach my $line ( @{ $aref } ) {
                $idx = sprintf "%015d", $start_idx++;
                print $fh $idx, $line;
            }
            $order_id++;
        }
    }
}

MCE::Loop::init {
    chunk_size => 'auto', max_workers => 3,
    gather => preserve_order($fh_out)
};

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

    foreach my $line (@{ $chunk_ref }) {
        $line =~ s/\r//g; chomp $line;

        my ($f1,$f2,$f3,$items,$f5,$f6,$f7) = split /\|/, $line;
        my @items_array = split /;/, $items;

        foreach my $item (@items_array) {
            push @buf, "|$item|$f1|$f2|$f3|$f5|$f6|$f7\n";
        }
    }

    MCE->gather($chunk_id, \@buf);

} $infile;

MCE::Loop::finish();
close $fh_out;


##</code><code>##

use strict;
use warnings;

use MCE::Loop;
use MCE::Candy;

my $infile  = shift or die "Usage: $0 infile\n";
my $newfile = 'output.dat';

open my $fh_out, '>', $newfile or die "open error $newfile: $!\n";

MCE::Loop::init {
    chunk_size => 'auto', max_workers => 8,
    gather => MCE::Candy::out_iter_fh($fh_out),
    init_relay => 1
};

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

    foreach my $line (@{ $chunk_ref }) {
        $line =~ s/\r//g; chomp $line;

        my ($f1,$f2,$f3,$items,$f5,$f6,$f7) = split /\|/, $line;
        my @items_array = split /;/, $items;

        foreach my $item (@items_array) {
            push @lines, "$item|$f1|$f2|$f3|$f5|$f6|$f7\n";
        }
    }

    my $idx = MCE::relay { $_ += scalar @lines };
    my $buf = '';

    foreach my $line ( @lines ) {
        $buf .= sprintf "%015d|%s", $idx++, $line
    }

    MCE->gather($chunk_id, $buf);

} $infile;

MCE::Loop::finish();
close $fh_out;