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  # Verify $prev is NON-empty
  if ($prev) { ... }

  # straightforward approach: track whether this
  # is the first iteration or not explicitly.
...
      }
      $prev = $elem;
  }

  # note that this alters @array
  my $prev = shift @array;
...
      push @result, compute( $elem, $prev );
      $prev = $elem;
  }

  my ( $prev, @rest ) = @array;
  foreach my $elem ( @rest )
...
      push @result, compute( $elem, $prev );
      $prev = $elem;
  }

  my $prev = $array[0];
  foreach my $elem ( @array[ 1 .. $#array ] )
...
      push @result, compute( $elem, $prev );
      $prev = $elem;
  }

  # note that we want $i to stop one short of $#array,
  # since we access $array[$i+1].
...
  {
      push @result, compute( $array[$i+1], $array[$i] );
  }