gistfile1.txt

use strict;
use warnings;

use Benchmark 'timethis';

timethis( -5, sub { f(18) }, 'fibonacci' );

sub fib {
    return $_[0] < 2 ? $_[0] : fib($_[0]-1) + fib($_[0]-2)
}

# Subroutine f(n) handles the negative arguments: F(-n) = F(n)*(-1)^(n+1)
sub f {
    if ($_[0] < 0) {
        return $_[0] % 2 ? fib(-$_[0]) : -fib(-$_[0]);
    } else {
        return fib($_[0]);
    }
}