Oracle 18c

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create or replace PROCEDURE ETA_JSON_SERIALIZE (
  json_in    IN     JSON_ELEMENT_T,
  can_str    IN OUT VARCHAR2,
  object_key IN     VARCHAR2 DEFAULT NULL
)
IS
BEGIN
  IF json_in.is_Object() THEN
    DECLARE
      l_object JSON_OBJECT_T := TREAT(json_in AS JSON_OBJECT_T);
      l_keys   JSON_KEY_LIST := l_object.get_Keys();
    BEGIN
      FOR i IN 1 .. l_keys.COUNT LOOP
        can_str := can_str || '"'||UPPER(l_keys(i))||'"';
        ETA_JSON_SERIALIZE(l_object.get(l_keys(i)), can_str, l_keys(i));
      END LOOP;
    END;
  ELSIF json_in.is_Array() THEN
    DECLARE
      l_array JSON_ARRAY_T := TREAT(json_in AS JSON_ARRAY_T);
    BEGIN
      FOR i IN 0 .. l_array.get_size -1 LOOP
        IF i > 0 AND object_key IS NOT NULL THEN
          can_str := can_str || '"'||UPPER(object_key)||'"';
        END IF;
        ETA_JSON_SERIALIZE(l_array.get(i), can_str);
      END LOOP;
    END;
  ELSIF json_in.is_Scalar() THEN
    can_str := can_str || json_in.to_String();
  END IF;
END;
/
SELECT * FROM USER_ERRORS;
DECLARE
   can_str     VARCHAR2(32767);
   l_element   JSON_ELEMENT_T;
   l_doc       CLOB :=  '{
    "delivery": {
        "approach": "SomeValue",
        "packaging": "SomeValue",
        "dateValidity": "2020-09-28T09:30:10Z",
        "exportPort": "SomeValue",
        "countryOfOrigin": "EG",
        "grossWeight": 10.50,
        "netWeight": 20.50,
        "terms": "SomeValue"
    },
    "taxTotals": [
        {
            "taxType": "T1",
            "amount": 477.54
        },
        {
            "taxType": "T2",
            "amount": 365.47
        },
        {
            "taxType": "T3",
            "amount": 60.00
        }
    ],
    "totalAmount": 5191.50,
    "extraDiscountAmount": 5.00,
    "totalItemsDiscountAmount": 14.00
}';
BEGIN 
   ETA_JSON_SERIALIZE(JSON_ELEMENT_T.parse( l_doc ), can_str);
   DBMS_OUTPUT.PUT_LINE(can_str);
END;
/
1 rows affected

dbms_output:
"DELIVERY""APPROACH""SomeValue""PACKAGING""SomeValue""DATEVALIDITY""2020-09-28T09:30:10Z""EXPORTPORT""SomeValue""COUNTRYOFORIGIN""EG""GROSSWEIGHT"10.50"NETWEIGHT"20.50"TERMS""SomeValue""TAXTOTALS""TAXTYPE""T1""AMOUNT"477.54"TAXTOTALS""TAXTYPE""T2""AMOUNT"365.47"TAXTOTALS""TAXTYPE""T3""AMOUNT"60.00"TOTALAMOUNT"5191.50"EXTRADISCOUNTAMOUNT"5.00"TOTALITEMSDISCOUNTAMOUNT"14.00