is there any overhead to using Tie:LLHash over using the array?

It depends on what you mean by "overhead". Certainly, there's overhead involved in loading a module. There's overhead involved in installing a module. Depending on how the module is implemented, there's likely overhead involved in using the module. Whether that overhead is worth it or not depends on a lot of factors. But personally, I would stick with a plain dual hash/array approach unless there was a specific reason not to.