Perl
====
<!-- language: lang-perl -->

    $b = ($i = <>) - ~-$i;
    {
    	($b, $a)=($a + $b, $b);
    	redo if --$i
    }
    print "$a\n";

This takes an input from STDIN and prints the n-th Fibonacci number.

`($i = <>)` initializes `$i` with the input. Then `$b` is set to `1` by a bit of bit manipulation and subtraction. The "magic" here is done by the `redo` operator. It makes it possible to reevaluate the same block, without the need of loop. The calculation of `$a` and `$b` is redone until the subtraction of one by `$i` evaluates to `False` i.e. `0`. I think that the `print` statement in the end is self-explanatory.