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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In reply to Re^3: hash of hashes and memory space... by ikegami
in thread hash of hashes and memory space... by bcarroll

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