Why does Autovivification occur with keys() and not %{..}?

Question

This is a subtlety I found with keys().

$ perl -e 'use warnings; use strict; my $d = { "ab" => 1 }; my @e = keys(%{$d->{cd}});'

$ perl -e 'use warnings; use strict; my $d = { "ab" => 1 }; my %e = %{$d->{cd}};'
Can't use an undefined value as a HASH reference at -e line 1.

I am most puzzled as to why the first snippet would not give an dereferencing error. When I use Data::Dumper, it becomes clear that in the first snippet, $d->{cd}, is autovivified to be {}.

Why does keys need to autovivify? I tried reading the perldoc for it, could not find a satisfying answer. keys does not set an alias ($_, etc) so there is no need for perl to think $d->{cd} needs to be in lvalue context, is there? (I understand if the expression needs to be in lvalue context autovivification happens as explained here.

A relevant post.

Answer 1

Note that keys can indeed be an lvalue (setting the hash's expected number of elements).

But even if keys itself isn't used in an lvalue context, it has a side effect of resetting a hash's iterator.

So it does modify the hash and so gives the dereference an lvalue context, which makes it autovivify.