($str, $substr, $newsubstr) = @ARGV;
#while ( -1 != ($current = rindex($str, $substr))) {
#    substr $str, $current, length($substr), $newsubstr;
#}
$i = 0;

while ( -1 != ($current = index($str, $substr, $i)) ) {
    substr $str, $current, length($substr), $newsubstr;
    $i = $current + length($newsubstr);
}

$\ = $/;

print $str;