if ($proc_pid = fork) {
  # parent
}
elsif (defined $proc_pid) {
  exec $command, @args;
  die "can't exec $command @args: $!";
}
else {
  die "can't fork: $!";
}

if ($count_pid = fork) {
  # parent
}
elsif (defined $count_pid) {
  opendir DIR, $dir_to_be_watched
    or die "can't open $dir_to_be_watched: $!";
  while (1) {
    # UPDATED -- who cares WHAT the files are?
    my $files = -2;  # -2 for . and ..
    $files++ while defined readdir DIR;
    last if $files == $N;
    rewinddir DIR;
  }
  kill -TERM => $proc_pid;
  exit;
}
else {
  die "can't fork: $!";
}

# parent can go on its merry way
[download]

You may want to make that:     last if $files >= $N; or even     last if ++$files >= $N while readdir DIR; Otherwise, the directory could jump from <N to >N files and your script would miss it.

Hmm, good point. Oh, but your second snippet is broken (since Perl doesn't allow multiple post-expression control modifiers).

++$files >= $N and last while defined readdir(DIR);
[download]

And keep that defined in there. The files "0" and "" are valid, you know.

japhy -- Perl and Regex Hacker