`perldoc perlref` will help you.

%hash = (
     'a' => { 'aa' => [0,1,2] , 'ab' => [3,4,5] },
     'b' => { 'ba' => [6,7,8] , 'bb' => [9,10,11] },
);

my %a_hash = %{ $hash{a} };
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