When an undefined variable is dereferenced, it gets silently upgraded to an
array or hash reference (depending of the type of the dereferencing). This
behavior is called autovivification and usually does what you mean (e.g.
when you store a value) but it's sometimes 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.
