cat fatset.chunk* > fatset

##</code><code>##

{
  local $/;  # sets input_record_separator to "undef"
  open(O,">fatset");
  for my $f (sort <fatset.chunk*>) {
     open(I,$f);
     $_ = <I>;  # reads entire chunk at once.
     print O;
  }
  close O;
}