As your output shows, the same process is handling all the tasks. That's because the parent is forking a child but then continuing the work itself.

You are missing the statement that causes the parent to skip out of the loop. See the doc for Parallel::ForkManager, which states (emphasis added):

• Next, use $pm->start to do the fork. $pm returns 0 for the child process, and child pid for the parent process (see also "fork()" in perlfunc(1p)). The "and next" skips the internal loop in the parent process.

The following example shows the difference:

use strict; use warnings; use feature 'say';
use Parallel::ForkManager;
use Time::HiRes qw/ time /;

$|++;

my $forker = Parallel::ForkManager->new(4);
say "v.1";
for ( 0 .. 9 ) {
    my $pid = $forker->start;
    if ( $pid ) {
        say "1 $$ $_ start: " . time;
        sleep 1;
    }
    $forker->finish;
}

my $forker2 = Parallel::ForkManager->new(4);
say "v.2";
for ( 0 .. 9 ) {
    my $pid = $forker->start and next;
    say "2 $$ $_ start: " . time;
    sleep 1;
    $forker->finish;
}

__END__
[download]

Output:

$ perl 1187274.pl

v.1
1 19015 0 start: 1491486225.44169
1 19015 1 start: 1491486226.44244
1 19015 2 start: 1491486227.44318
1 19015 3 start: 1491486228.44402
1 19015 4 start: 1491486229.44495
1 19015 5 start: 1491486230.44586
1 19015 6 start: 1491486231.447
1 19015 7 start: 1491486232.44791
1 19015 8 start: 1491486233.44877
1 19015 9 start: 1491486234.4497
v.2
2 19026 0 start: 1491486235.45079
2 19027 1 start: 1491486235.45113
2 19028 2 start: 1491486235.45138
2 19029 3 start: 1491486235.4517
2 19030 4 start: 1491486237.45457
2 19031 5 start: 1491486237.45492
2 19032 6 start: 1491486237.45516
2 19033 7 start: 1491486237.45543
2 19035 9 start: 1491486239.45953
2 19034 8 start: 1491486239.45992
[download]

Hope this helps!