our %kid;

sub forkus {
    my $task = shift;     # it's a hash ref 
    defined(my $cpid = fork) or warn $! and return;
    $kid{$cpid} = $_->id, return $cpid if $cpid;
    exec $_->cmd;
    exit -1;
}
forkus($_) for (@first_tasks);

while (%kid) {
    my $cpid = wait;    # sleep until somebody's done
    forkus(next_task($cpid)) if more($cpid); # suitably defined
    delete $kid{$cpid};
}