Lexically disable autovivification.
When an undefined variable is dereferenced, it gets silently upgraded to an array or hash reference (depending of the type of the dereferencing). This behaviour is called _autovivification_ and usually does what you mean (e.g. when you store a value) but it may be unnatural or surprising because your variables gets populated behind your back. This is especially true when several levels of dereferencing are involved, in which case all levels are vivified up to the last, or when it happens in intuitively read-only constructs like 'exists'.
This pragma lets you disable autovivification for some constructs and optionally throws a warning or an error when it would have happened.