Humour me and post your results from running this simplified version of your code with arg 4(assuming you have 4 cores?) & 50:

#!/usr/bin/env perl
use strict;
use warnings;
use Math::BigInt;
use Parallel::ForkManager;
use Time::HiRes qw ( time );
use feature qw(say);

my $processes = shift;
my $pm = Parallel::ForkManager->new( $processes );

say qq(processes: $processes);

my $start=time;

for my $number ( 1 .. 50 ) {
    $pm->start( $number ) and next;
    my $factorial = Math::BigInt->bfac( 2000 ) for 1 .. 10;
    $pm->finish( 0 );
}
$pm->wait_all_children;

say qq(fork: ), time - $start;
[download]