Oracle 21c

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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 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
    IF json_in.is_String() THEN
        can_str := can_str || json_in.to_String();
    ELSIF json_in.is_Number() THEN
        can_str := can_str || '"' || TO_CHAR(json_in.to_Number(), 'FM999999990.00') || '"';
    END IF;
  END IF;
END;
/
SELECT * FROM USER_ERRORS;
DECLARE
   can_str     VARCHAR2(32767);
   l_doc       CLOB := '{
    "taxTotals": [
        {
            "taxType": "T1",
            "amount": 477.54
        },
        {
            "taxType": "T2",
            "amount": 365.47
        },
        {
            "taxType": "T3",
            "amount": 60.00
        },
        {
            "taxType": "T4",
            "amount": 75.99
        },
        {
            "taxType": "T5",
            "amount": 214.71
        },
        {
            "taxType": "T6",
            "amount": 120.00
        }
    ],
    "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:
"TAXTOTALS""TAXTOTALS""TAXTYPE""T1""AMOUNT""477.54""TAXTOTALS""TAXTYPE""T2""AMOUNT""365.47""TAXTOTALS""TAXTYPE""T3""AMOUNT""60.00""TAXTOTALS""TAXTYPE""T4""AMOUNT""75.99""TAXTOTALS""TAXTYPE""T5""AMOUNT""214.71""TAXTOTALS""TAXTYPE""T6""AMOUNT""120.00""TOTALAMOUNT""5191.50""EXTRADISCOUNTAMOUNT""5.00""TOTALITEMSDISCOUNTAMOUNT""14.00"