use POSIX ":sys_wait_h";
my $child = fork;
if ($child) { # parent case
    my $res;
    while (not ($res = waitpid($child, WNOHANG))) {
        sleep 5;

        # check file list here
        # ...
    }
    if ($res == -1) {
        warn "fatal child error\n";
        exit;
    }
}
else { # child case
    # insert job processing here
    exit; # child must explicitly exit otherwise both would continue executing this
}
# reach here if job processing completed successfully