my @digits = /./sg;

my @next = (1,2,3,4,5,6,7,8,9,0);
my @prev = (9,0,1,2,3,4,5,6,7,8);

my $longest_start = my $start = 0;
my $longest_len   = my $len   = 1;
my $slope         = 0;
for (1..$#digits) {
   my $new_slope =
      $digits[$_-0] == $next[$digits[$_-1]] ? +1 :
      $digits[$_-0] == $prev[$digits[$_-1]] ? -1 :
      0;

   if ($new_slope) {
      if ($new_slope == $slope) {
         ++$len;
      } else {
         $start = $_ - 1;
         $len   = 1;
      }
   }

   $slope = $new_slope;

   if ($len > $longest_len) {
      $longest_start = $start;
      $longest_len   = $len;
   }
}

my $longest = substr($_, $longest_start, $longest_len+1);