There are other ways of mutating a string than just appending to the end. For example:

$n=1e3; 
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){         
            substr $s, $i, 1, chr(1);
         }                                 
     ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
             $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
    Rate    b    a
b  811/s   -- -85%
a 5525/s 581%   --

$n=1e4;
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){        
             substr $s, $i, 1, chr(1);
        }                                  
    ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
            $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
    Rate     b     a
b 13.0/s    --  -98%
a  556/s 4173%    --

$n=1e5; 
cmpthese -1,{
    a=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){         
            substr $s, $i, 1, chr(1);
        }                                  
    ],
    b=>q[
        my$s = chr(0)x$n; 
        for my $i ( 0..length($s)-1){ 
            $s = substr($s,0,$i-1) . chr(1) . substr $s,$i+1;
        } 
    ]
};;
            (warning: too few iterations for a reliable count)

     s/iter      b      a
b      12.5     --  -100%
a 1.77e-002 70277%     --
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