Documented in exists (excerpt):

if (exists $ref->{A}->{B}->{$key})  { }
if (exists $hash{A}{B}{$key})       { }
...
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Although the most deeply nested array or hash element will not spring into existence just because its existence was tested, any intervening ones will. Thus $ref->{"A"} and $ref->{"A"}->{"B"} will spring into existence due to the existence test for the $key element above. This happens anywhere the arrow operator is used, ...