Rate        split1       substr1         subst subst_inplace
split1        2.93/s            --          -72%         -100%         -100%
substr1       10.3/s          253%            --          -99%          -99%
subst          901/s        30682%         8623%            --          -24%
subst_inplace 1179/s        40176%        11313%           31%            --
ok 1
ok 2
ok 3
1..3

##</code><code>##

sub subst_inplace
{
    my ($s1, $s2) = @_;

    use bytes;
    $$s1 =~ s/(\0)/substr $s2, $+[0]-1, 1/eg;
}