Not that it matters much to me, but for the 71st number I get a round-off error using Binet's formula.

sub fib {int(((((1+sqrt(5))/2)**$_[0])/sqrt(5))+.5)}

sub fib2 {
    my ($n) = @_;

    my ($x, $y) = (1, 1);
    ($x, $y) = ($y, $x + $y)
        for 3 .. $n;

    return $y;
}

for (my $n = 1; 1; $n++) {
    my ($x, $y) = (fib($n), fib2($n));
    if ($x != $y) {
        print "$n: $x $y";
        last;
    }
}

__END__
71: 308061521170130 308061521170129
[download]

Regards,
ihb