It might do you some good to read through I know what I mean. Why don't you?. For example, you ask us what library you can use instead of bigint but don't tell us what you are trying to do. And please post code when you have it.

In any case, assuming you want to use bigint in a highly localized fashion, you can either use a use and no block to control how the packages are used or you can just wrap the code that needs bigint in a block, thus localizing the use. Note that the answer to your question is right in the code examples in the bigint documentation - Perl documentation is your friend.