sub Fib($)
{
  my $index = shift;
  return 0 if 0 == $index;
  return 1 if 1 == $index;
  return Fib( $index - 1 ) + Fib( $index - 2 );
}

##</code><code>##

sub Fib($)
{
  my $index = shift;
  my $acc = 0;
  my $tmp = 1;

  while( $index-- )
  {
    # Swap the accumulator and the temp.
    # Use an Xor swap for fun and to amaze your friends.
    $acc^=$tmp^=$acc^=$tmp;

    # Add the two together and store it in the acc.
    $acc += $tmp;
  }

  return $acc;
}

##</code><code>##

{
  my @cache = (0, 1); # Localized so only &Fib sees it.

  sub Fib($)
  {
    my $index = shift;
    return $cache[$index] if defined $cache[$index];
    $index -= $#cache;
    push @cache, $cache[-1] + $cache[-2] while $index--;
    return $cache[-1];
  }
}