%hash = (
           key1 => { key1_1 => ["value1_1",value1_2"],
                     key1_2 => "w00t!",
                   },
           key2 => { key2_1 => "wakkawak!", 
                     # key2_2 => $hashref->{key1}{key1_1},   # postpone, can't read it yet... 
                   },
           );
$hash{key2}{key2_2} = $hashref->{key1}{key1_1};  # now you can.