$s='a'x1e6; 
cmpthese -1, {
    a => q[ 
        $s =~ m[(.)(?{$c=pos();})]g;
    ],
    b => q[ 
        $s =~ m[(.)]g;
    ], 
};;
       Rate       a       b
a    3924/s      --   -100%
b 4206322/s 107088%      --