#!/usr/bin/env perl
use strict;
use warnings;
use Time::HiRes qw(sleep);

my $workercount = 0; # Counts active workers

SIG{CHLD} = sub {
    # Child exit detected
    $workercount--; # remove child from "active" count
    return;
}

while(1) {
    if($workercount < 20) {
        my $pid = fork();
        if($pid) {
            # parent
            $workercount++; # forked a new child
        } else {
            # child
            my $cmd = 'perl myworkerscript.pl';
            eval {
                exec($cmd);
            };
            exit(0);
        }
    } else {
        sleep(0.5);
    }
}