use Benchmark 'cmpthese';

my ($x,$y)=(1,2);

cmpthese(-2,
         {list  =>sub {
                       ($x,$y)=($y,$x) #list assign, slow
                      },
          scalar=>sub{
                      #three lines. fast.
                      my $tmp=$x;
                      $x=$y;
                      $y=$tmp
                     }
         }
        );

__END__

Benchmark: running list, scalar, each for at least 2 CPU seconds...
      list:  ( 2.34 usr +  0.00 sys =  2.34 CPU) @ 550148.10/s (n=1288997)
    scalar:  ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 878828.44/s (n=1854328)
           Rate   list scalar
list   550148/s     --   -37%
scalar 878828/s    60%     --