para.pl

#!/usr/bin/env perl
# References:
# - http://stackoverflow.com/a/800105/492631

use warnings;
use strict;

# fork processes
my $num_cores = 4;
my @children = ();
foreach my $core (1..$num_cores) {
    my $child = fork();
    if (! $child) { # I am child
        print STDERR "running child $core\n";
        exec("sleep 2 && echo $core > $core");
        exit 0;
    }
    push @children, $child;
    next;
}

print "waiting on children\n";
foreach my $child (@children) {
    waitpid( $child, 0 );
}

parajobs.pl

#!/usr/bin/env perl
# References:
# - http://stackoverflow.com/a/800105/492631

use warnings;
use strict;

# fork processes
my $num_cores = 4;
my $num_jobs = 15;
my @children = ();
my $job = 0;

foreach my $core (1..$num_cores) {
    if ($job < $num_jobs) {
        my $child = fork();
        if (! $child) { # I am child
            #print STDERR "running child $core\n";
            exec("sleep 1 && echo $job > $job");
            exit 0;
        }
        push @children, $child;
        $job = $job + 1;
        next;
    }
}

while (@children) {
    my $old_child = shift @children;
    waitpid( $old_child, 0 );
    print "child finished\n";

    if ($job < $num_jobs) {
        my $new_child = fork();
        if (! $new_child) { # I am child
            exec("sleep 1 && echo $job > $job");
            exit 0;
        }
        $job = $job + 1;
        push @children, $new_child;
    }
}

The output of running this:

$ time perl para.pl
running child 1
running child 2
running child 3
waiting on children
running child 4
perl para.pl  0.02s user 0.03s system 2% cpu 2.031 total

so it took 2 seconds total, and each subprocess took 2 seconds.

$ head [0-9]
==> 1 <==
1

==> 2 <==
2

==> 3 <==
3

==> 4 <==
4

Great! What about if you have more jobs than cores? In that case we need to start up next jobs as spots become available.

So parajobs.pl uses a FIFO queue to run through all the jobs. It's not as optimal as a pool, since some jobs might finish before a job ahead of them.

I tested this, and 15 1-second jobs with 4 cores took 4.226 total seconds.