And one can even make it simpler, knowing that Tcl will internally shimmer the value to a dict as soon as a dict primitive is used:

set big_data_strucutre "
                          key1 value1
                          key2 value2
                          key3 {item1 item2 item3}
                          key4 value4
                          key5 {item1 {item21 item22}}
                          key6 value6"