You can't nullify an `MCStringRef`, to delete it you want to use `MCValueRelease`. 
Anyway here, it was done like this to avoid a string copy: `r_boundary` is made to point to the memory of `t_values[i]`, and then `t_values[i]` is set to NULL so that it won't get `free`'d during the memory release in the final loop of the function.
You merely have to update the signature to having `MCStringRef` and using `MCValueRetain` there, or keep the `strdup(...)` and remove the `nil` assignment.