It's not the function that is causing the key to be created as has been explained in this thread. Autovivification also allowed me to solve the problem at Migrating a 1NF table to multiple 2NF tables with more ease.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

That doesn't technically count as autovivification, though; that's undef being treated as 0 in a numeric context. Autovivification is the automatic creation of a reference to a new anonymous hash (or array) when required to implement a multilevel data structure.</pedant>

Update: And to out pedant myself, you don't necessarily have to have a multilevel structure; my $aref; push @{ $aref }, 1; is autovivification as well. The Uri article referenced from the node I mentioned below uses the phrasing automatically creating a reference of the appropriate type whenever a scalar containing undef is dereferenced. Better than what I used above.

Further update: The parent did have code using $hash{ $_ }++ and calling that autovivification. It's been removed so this now will make less sense. Ah well.