In addition to the perl idiom of using a list assignment to swap values without a temporary variables, there is the idiom using the XOR operator. Although this does take three statements and doesn't fully take advantage of perl's list assignments, and so is more useful in other languages.
$a ^= $b;
$b = $a ^ $b;
$a = $a ^ $b;
Update:Here's a contracted form of the above:
$a = $a ^ ($b = ($a ^= $b) ^ $b);
--
integral, resident of freenode's #perl
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