use Benchmark;
timethese( 100, {with => q{
             my %hash;
             keys( %hash ) = keys( %hash ) = 10000;
             foreach $i (1 .. 10000) {
                $hash{"key$i"} = $i;
             }
           },
           without => q{
              my %hash;
              foreach $i (1 .. 10000) {
                 $hash{"key$i"} = $i;
              }
           },
         });