Autovivification is the concept that a hash style data structure can make inferences about its internal structure as it is being created. For instance, creating a nested hash under one of the keys would normally require you to explicitly state that you wanted to create a hash there. Autovivification allows you to skip this step and create the nested structure on the fly.
Eg. a.b.c.d.e = 1 b,c,d,e get created on the fly, if not present