DB<5> no autovivification; $h->{a}{b}{c} = 42

  DB<6> x $h
0  HASH(0xb400007e0b2fd8b8)
   'a' => HASH(0xb400007e0b2ea7f8)
      'b' => HASH(0xb400007e0b2eac30)
         'c' => 42
  DB<7>