drop table xxx_square;

CREATE TABLE xxx_square
(   A VARCHAR2(100)
  , B VARCHAR2(100)
);
    
CREATE OR REPLACE PACKAGE xxx_p1
IS
  PROCEDURE xxx_p_square (A IN VARCHAR2, B OUT NOCOPY VARCHAR2);
END xxx_p1;
/

CREATE OR REPLACE PACKAGE BODY xxx_p1
IS
  PROCEDURE xxx_p_square (A IN VARCHAR2, B OUT NOCOPY VARCHAR2)
  AS
  BEGIN
    B:= A || A;
  END xxx_p_square;
END xxx_p1;
/

CREATE OR REPLACE TRIGGER xxx_tr_square BEFORE INSERT ON xxx_square FOR EACH ROW
BEGIN
    xxx_p1.xxx_p_square(:new.a, :new.b);
END xxx_tr_square;

INSERT INTO xxx_square (A)
                     VALUES
                       ('123456789012345');

SELECT *
FROM xxx_square;

CREATE OR REPLACE TRIGGER xxx_tr_square BEFORE INSERT ON xxx_square FOR EACH ROW
DECLARE

  tmp_b VARCHAR2(100);
  
BEGIN

    xxx_p1.xxx_p_square(:new.a, tmp_b);
    :new.b := tmp_b;
    
END xxx_tr_square;
/