use POSIX ':sys_wait_h';

my $max_children = 20;
my $cur_children = 0;

$SIG{CHLD} = sub {
   $cur_children-- while waitpid(-1, &WNOHANG) != -1;
   &spawn_children;
}

sub spawn_children {
   while ($cur_children < $max_children) {
      my $pid = fork;
      die "fork: $!" unless defined $pid;
      &child_process if !$pid;
      $cur_children++;
   }
}

sub child_process {
   # what the kid does
   exit 0;   # important
}

&spawn_children;