> Modifier o means that the first time a literal regexp with interpolations is encountered, the generated Regexp object is saved and used for all future evaluations of that literal regexp.

That is crystal clear to me. It means that on the next execution, the new values of the interpolation will be ignored; the regexp is now "baked" with the first ones.

Like this in C++:

  void fun(int arg)
  {
     static int once = arg;
  }

There is a function in POSIX for once-only initializations: pthread_once. C++ compilers for multithreaded environments emit thread-safe code to do something similar to pthread_once to ensure that even if there are several concurrent first invocations of the function, the initialization happens once.