No, the -1 says "any" process, not "all processes". Usually you use waitpid to wait for a specific pid (hence the name ;-). But in both cases, it will return upon detecting one terminated child process.

To create a throttle in the signal handler, you need to make the parent stop when your list of pids reaches ten elements (like you're doing). Then you need to make the signal handler routine (reapchild in the example above) shorten the list every time waitpid returns something other than -1. So:

sub reapchild
{
    while (waitpid(-1, &WNOHANG) > 0)
    {
        pop @pids;
    }
}
[download]

sub reapchild
{
    my ($pid, $index);
    PID:
    while (($pid = waitpid(-1, &WNOHANG)) > 0)
    {
        foreach my $i (0..$#pids)
        {
            if ($pids[$i] == $pid)
            {
                splice @pids, $i, 1;
                next PID;
            }
        }
        warn "Unexpected $pid returned!\n";
    }
}
[download]

HTH