use Benchmark 'cmpthese';

our $orig  = "a" x 20;
    $orig .= "<!-- TBROWS //-->";
    $orig .= "a" x 20;
    $orig .= "<!-- TBROWS //-->";
    $orig .= "a" x 20;

cmpthese(-1, {
    's///'  =>   'my $c = $orig; $c =~ {<!-- TBROWS //-->}{XXXX}g;',
    substr  =>   'my $c = $orig;
                  while ((my $i = index($c, "<!-- TBROWS //-->")) > -1) {
                    substr($c, $i, 17) = "XXXX";
                  }'
});

__END__
           Rate substr   s///
substr 240498/s     --   -37%
s///   381869/s    59%     --