my ($largest_index, $largest_value) = (-1E6, undef);
#make sure $largest_index starts below the lowest possible
#value you can get
for my $data (@some_big_long_list){
  my $this_index = index_from($data);
  if ($this_index > $largest_index){
     $largest_index = $this_index;
     $largest_value = $data;
  }
}
#do something with largest data