Hmmmm. I might be confused with functions being re-entrant and un-buffered I/O.

What happens in my process is that I'm not having any waitpid but instead polling to check if a new line has been printed on the stream.
Let me put the exact code since this might be something specific:

  $selector = IO::Select->new();
  $selector->add(*CMD_ERR, *CMD_OUT);

  while(@ready = $selector->can_read) {
    foreach my $fh (@ready) {
      if(fileno($fh) == fileno(CMD_ERR)) {
        my $line = scalar <CMD_ERR>;
        print $line;
      }
    }
  }
[download]

Basically if what your meaning by unbuffered is having lines on the stream which is not printed when the signal occur, then yes I very likely have.

UPDATE: I've tried this on two systems. Opensolaris and SLC4, both 32 bit. My perl version is: v5.8.4