Very interesting. The issue doesn't deal with defined. That can be checked by the following:

my @keys = qw(foo bar baz);
my %h;

grep '', @h{@keys};              # creates keys
1 for    @h{@keys};              # creates keys
map  '', @h{@keys};              # creates keys

my @a = @h{@keys};               # DOESN'T create keys
sort {$a <=> $b} @h{@keys};      # DOESN'T create keys
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I think it depends on creating a list for iterations