To answer your second question

$dd{'1'}{'2'}{'3'}="hi";
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is equivalent to

my %dd = (
    1 => {
        2 => {
            3 => 'hi'
            }
        }
);
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and %{$dd{'1'}} is equal to ( 2 => { 3 => 'hi' } ). To get the value you assign it to a scalar and then dereference the keys of that hash ref.

my $hash_ref1 = $dd{1};
print $hash_ref->{2}{3}, "\n";
# prints h1
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or if you are iterating over a structure

while ( my ( $key, $hash_ref ) = each ( %{ $dd{1} } ) {
    # $key == 2
    # $hash_ref == { 2 => { 3 => 'hi' }  }
}
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