Autovivify in this usage is a term coined to mean that perl automatically creates a variable the first time you talk about it. Vivify means to "bring to life" (or worse "to operate on something while it is alive"). Tack on "auto" and you get the general idea.

If you do not assign to a newly autovivified variable it will default to the undefined value (undef).

use strict and use warnings help.

The difference between (a) a hash key that exists and is defined, (b) a hash key that exists and is not defined, and (c) a hash key that does not exist - is a common point of confusion.

-Andrew.