Cómo inicializar una matriz en plsql

No necesita usar una matriz asociativa; una colección funcionará.

DECLARE
  TYPE RecType IS RECORD
  (
    value1   NUMBER,
    value2   NUMBER,
    value3   NUMBER
  );
  TYPE TblType IS TABLE OF RecType;
  TYPE TblOfTblType IS TABLE OF TblType;
  matrix   TblOfTblType := TblOfTblType();
BEGIN
  matrix.EXTEND(3);
  FOR i IN 1 .. matrix.COUNT LOOP
    matrix(i) := TblType();
    matrix(i).EXTEND(4);
    FOR j IN 1 .. matrix(i).COUNT LOOP
      matrix(i)(j).value1 := i;
      matrix(i)(j).value2 := j;
      matrix(i)(j).value3 := DBMS_RANDOM.VALUE;
    END LOOP;
  END LOOP;

  FOR i IN 1 .. matrix.COUNT LOOP
    FOR j IN 1 .. matrix(i).COUNT LOOP
      DBMS_OUTPUT.PUT( '[' || matrix(i)(j).value1
                    || ',' || matrix(i)(j).value2
                    || ',' || matrix(i)(j).value3 || ']' || CHR(11) );
    END LOOP;
    DBMS_OUTPUT.NEW_LINE;
  END LOOP;
END;
/