sub append_like_hell {
  my $id = shift;
  open my $log, '>>', FILE_NAME or die ...
  for (1..$NUM_LINES) {
    syswrite($log, sprintf(...)) or die "...";
  }
  close($log);
}

##</code><code>##

for (my $ofs = 0; $ofs < length $wbuf; ) {
  $ofs += (my $wr = syswrite $fh, substr $wbuf, $ofs or die "unable to write results");
}