$s = 'ab'x 10e6;;

cmpthese -1, {
    a => q[ 
        my $n = 0; 
        while( $s =~ m[ab]g ){
            $n++
        };
    ],
    b=>q[
        my $n = 0; 
        ++$n while $s =~ m[ab]g;
    ],
};;

  s/iter    a    b
a   2.31   -- -37%
b   1.45  59%   --