Benchmark: timing 1000000 iterations of perl_swap, xor_swap...
 perl_swap:  5 wallclock secs ( 4.87 usr +  0.00 sys =  4.87 CPU) 
             @ 205338.81/s (n=1000000)
  xor_swap:  4 wallclock secs ( 4.23 usr +  0.00 sys =  4.23 CPU) 
             @ 236406.62/s (n=1000000)