F(n) = floor((((1+sqrt(5))/2)**n) / sqrt(5)) + 0.5)

##</code><code>##

my $PHI = (1 + sqrt(5)) / 2;
sub fib ($) {int($PHI ** $_[0] / sqrt(5) + 0.5)}
say fib $_ for 0 .. 10;
__END__
0
1
1
2
3
5
8
13
21
34
55