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;
    },
}); 

##</code><code>##

Benchmark: running a, b for at least 1 CPU seconds...
         a:  1 wallclock secs ( 1.03 usr +  0.01 sys =  1.04 CPU) @ 124.04/s (n=129)
         b:  1 wallclock secs ( 1.08 usr +  0.01 sys =  1.09 CPU) @ 191.74/s (n=209)
   Rate    a    b
a 124/s   -- -35%
b 192/s  55%   --