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.