use Benchmark;
Benchmark::cmpthese(shift, {
    a => q{
        $a[++$i] = "";
        $a[$i] .= "a" for 1 .. 10000;
    },
    b => q{
        $b[++$j] = "b" x 10000;
        $b[$j] = "";
        $b[$j] .= "b" for 1 .. 10000;
    },
});