jeffp@hut [3:04pm] ~ #103> perl -MBenchmark
timethese(1_000_000, {
  'swap' => q{ ($a,$b) = ($b,$a) },
  'three' => q{ { my $c = $a; $a = $b; $b = $c } },
});
Benchmark: timing 1000000 iterations of swap, three...
      swap: 17 wallclock secs ( 5.04 usr +  0.00 sys =  5.04 CPU) @ 198449.61/s (n=1000000)
     three: 29 wallclock secs ( 5.96 usr +  0.01 sys =  5.97 CPU) @ 167539.27/s (n=1000000)