A "hash of hash" is really a "hash of references to hashes".

$h{foo}{bar} = 123;
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is short for

$h{foo}->{bar} = 123;
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and

${ $h{foo} }{bar} = 123;
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Something to try:

my %a;
my %b;
$a{foo} = \%b;
$a{foo}{bar} = 123;
print("$b{bar}\n");
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