sub order {
  state $p = 0;
  my $t = $p;
  $p = $^N;
  return $^N - $t == 1;
  }

$s =~ s{
  (?<! \d) ($n) \K (?{   order() })
  (?: $sep ($n)  (?(?{ ! order() }) (*F))){2,}
  }
  {-$^N}xmsg;