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

sub Fib($)
{
...

  return $acc;
}

{
  my @cache = (0, 1); # Localized so only &Fib sees it.
...
    return $cache[-1];
  }
}