$ perl -e 'use Benchmark(cmpthese);
cmpthese(1000000, {
     "a" => sub {
        $_ = "this that this those this these.";
        $a = () = /this/g;
    },
     "b" => sub {
        $_ = "this that this those this these.";
        $a++ while(s/this//);
    },
     "c"  => sub {
         $_ = "this that this those this these.";
         $a = s/this/this/g;
    }
});'
      Rate    a    b    c
a 282406/s   -- -28% -28%
b 390778/s  38%   --  -1%
c 393391/s  39%   1%   --