use strict;
use warnings;
use feature 'say';
use POSIX ":sys_wait_h";
use Time::HiRes qw(usleep);

my $maxProcs = 20; # Maximum number of processes to run concurrently
my $countProcs = 0; # Keep track of how many are running
my $finished = 0; # Flag variable
my $startTime = time(); # Going to keep forking until now + 30

sub main {
        while ($countProcs < $maxProcs) {
                my $child = fork();
                die "Cannot fork: $!" unless defined($child);
                if ($child == 0) {  # Child process
                        doStuff();
                } else {
                        $countProcs++;
                }
        }
        usleep(50); # The main process sleeps so not burning up cpu cycles
                    # in a wait loop
}

sub doStuff {
        my $sleepTime = rand() * 10+3; #3-13 seconds
        printf("%i doing work for %.02f seconds\n",$$,$sleepTime);
        sleep $sleepTime;
        exit;
}

while (not $finished) {
        main();
        #https://www.freebsd.org/cgi/man.cgi?query=waitpid&manpath=SuSE+Linux/i386+11.3
        my $reaped =
                waitpid(-1,WNOHANG);  # Wait for any child process without blocking
        if ($reaped) {
                say "$reaped reaped";
                $countProcs--;
        }
        $finished = 1 if (time() > $startTime + 30);
}

while (wait() != -1) {
        usleep(50);
}