You specify that the place s is const, not the value it keeps. You can still do:

    #include <string>
    #include <iostream>

    // not your code
    void innocentF(std::string const& s) {
      (const_cast<std::string&>(s)) += std::string(" I'm evil.");
    }

    // your code
    int main() {
      std::string const s("hello world.");
      innocentF(s);
      std::cout << s << std::endl; // hello world. I'm evil.
      return 0;
    }