my ($f, $fn) = (0, 1);
($f, $fn) = ($fn, $f + $fn) for 1 .. 25;
print $f;

##</code><code>##

my $PHI = (1 + sqrt(5)) / 2;
Fibonnaci(N) = ($PHI ** $N - (1 - $PHI) ** $N) / sqrt(5);

##</code><code>##

Fibonnaci(N) = int(0.5 + $PHI ** $N / sqrt(5))