my @sorted_lines
    = map { $_->[1] }
      sort { $a->[0] <=> $b->[0] }
      map { [(substr $_,2,8) => $_] }
      <DATA>;

__DATA__
1&20070102&string3&string4....
2&20070101&string3&string4....
3&20061212&&string3&string4....