substr is lvaluable:

$string = "pre replace post";
$i = 4;
$j = 11;
$length = $j - $i;
substr($string, $i, $length) = "x" x $length;
print "$string\n";
[download]

Or to avoid using $length twice:

$string = "pre replace post";
$i = 4;
$j = 11;
substr($string, $i, $j - $i) =~ tr/x/x/c;
print "$string\n";
[download]

But that's probably gratuitous obfuscation.