my $children_wanted = 6;
my @job_queue = map { 'sleep ' . int rand 100 } 1 .. 140;
my %job_of = ();

while ( $children_wanted < scalar keys %job_of
        && @job_queue ) {
    my $job = go( shift @job_queue );
    $job_of{ $job->{pid} } = $job;
    wait_on_jobs();

    sleep 5;
}

wait_on_jobs() while %job_of;

my $too_long = whatever();

sub wait_on_jobs {
    foreach my $job ( values %job_of ) {
        my $pid = $job->{pid};
        if ( $pid == waitpid $pid, WNOHANG ) {
            delete $job_of{ $pid };
        }
        
        if ( time() - $job->{start} > $too_long ) {
            kill 'TERM', $pid;
        }
    }
    
    return;
}

sub go {
    my ( $cmd ) = @_;

    my $pid = fork;
    if ( ! defined $pid ) {
        die "Can't fork: $!";
    }
    if ( $pid ) {
        # parent
        return { pid => $pid, cmd => $cmd, start => time() };
    }
    else {
        # child
        # close and exec and stuff
    }
    
    die 'unreachable code reached';
}