autovivification won't help because the OP isn't talking about the what we normally call autovivificaion (the creation of a referenced variable through dereference of an undefined value).

$ perl -e'
   my %h;
   1 for $h{k};
   CORE::say 0+keys(%h);
'
1

$ perl -e'
   no autovivification;
   my %h;
   1 for $h{k};
   CORE::say 0+keys(%h);
'
1
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