Autovivification. Because you have to dereference $questions{4} to get to $questions{4}{foo}, the reference $questions{4} must be created.

Update: Note that if the not-already-existing reference is not dereferenced, but is just read, no autovivification goes on, so your code would work as you'd like if you did:

 for (1 .. 4) {
    my $foo = $questions->{$_} && $questions->{$_}{foo} || "daklfjsdal
+k";
}
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