Well, your use case probably isn't as clearly stated as you think it is. If you're saying that you need to quickly check to see if a given anonymous array has been examined, why not reserve the zeroth element of each array for a "I've been examined!" indicator?

If, on the other hand, you mean that you need to check the entire set of arrays to see which ones have been examined, then you're still going to traverse the whole set of arrays, and a hash doesn't save you, because the long time isn't in the lookup, it's in the traversal.