# straightforward approach: track whether this
  # is the first iteration or not explicitly.
  my $is_first = 1;
  my $prev;
  foreach my $elem ( @array )
  {
      if ( $is_first )
      {
          $is_first = 0;
      }
      else
      {
          push @result, compute( $elem, $prev );
      }
      $prev = $elem;
  }