The "typo" was just a convenient way of illustrating a "hash key that got messed up somehow". This can happen in less trivial ways. E.g. a bug in a regex can send you down that path. Still, to this one could reply, well, fine, bugs happen, what's new? The reason for my bringing all this up is to point out that in the presence of autovivification such mangled keys lead to two errors. One is the familiar one: the fetching of nonexistent values leaves one with variables at the receiving end that are erroneously set to undef. It's the second one is the one that blindsides newcomers: the creation of new hash keys as a side effect. Programmers, I think, are more sensitized to the former than to the latter. As I said elsewhere, the most insidious bugs are those whose possibility one is not even aware of. Moreover, in my experience, these autovivification bugs would often manifest themselves far away (in the program's logic) from where the error happened.