SQL> get int
  1  CREATE OR REPLACE PROCEDURE data_repl
  2     (IN_TABLE IN VARCHAR2,
  3     IN_FLAG IN VARCHAR2)
  4     IS
  5     SQLSTRING VARCHAR2(500);
  6     V_TABLE VARCHAR2(100);
  7     V_FIELD VARCHAR2(100);
  8  V_CTR NUMBER(3) := 0;
  9  BEGIN
 10  V_TABLE := IN_TABLE||'_BACK';
 11  SQLSTRING := 'INSERT INTO ' || V_TABLE || ' VALUES(' ;
 12  FOR TNAME IN (SELECT COLUMN_NAME FROM USER_TAB_COLUMNS WHERE TABLE_NAME = IN_TABLE) LOOP
 13     V_CTR := V_CTR +1;
 14     IF V_CTR = 1 THEN
 15             V_FIELD := ':NEW.' || TNAME.COLUMN_NAME;
 16     ELSE
 17             V_FIELD := ',:NEW.' || TNAME.COLUMN_NAME;
 18     END IF;
 19     SQLSTRING := SQLSTRING || V_FIELD;
 20  END LOOP;
 21  SQLSTRING := SQLSTRING || ',''' || in_flag || ''')';
 22  -- INSERT INTO TMP VALUES (SQLSTRING);-- THIS WORKS FINE
 23  EXECUTE IMMEDIATE SQLSTRING; -- THIS GIVES ERROR
 24  DBMS_OUTPUT.PUT_LINE(sqlstring);
 25* END;
 26  /

Procedure created.

SQL> 
SQL> CREATE OR REPLACE TRIGGER TRAGA
  2  AFTER INSERT ON DEPT
  3  FOR EACH ROW
  4  BEGIN
  5  DATA_REPL('DEPT','F');
  6  END;
  7  /

Trigger created.

SQL> INSERT INTO DEPT VALUES
  2  ('38','DD','DD');
INSERT INTO DEPT VALUES
            *
ERROR at line 1:
ORA-00600: internal error code, arguments: [15212], [3], [], [], [], [], [], []
ORA-06512: at "MAG.DATA_REPL", line 23
ORA-06512: at "MAG.TRAGA", line 2
ORA-04088: error during execution of trigger 'MAG.TRAGA'.

this is proved, when i commented execute immedidate 
and used dbms_output to display just sqlstring (before executing). then the output of the trigger/procedure was something like this.
 
SQL> INSERT INTO DEPT VALUES
  2  ('38','DD','DD');
INSERT INTO DEPT_BACK VALUES(:NEW.DEPTNO,:NEW.DNAME,:NEW.LOC,'F')

1 row created.
<i>dynamic sql treats :NEW.deptno etc (pseudo records) etc as BIND VARIABLES.</i> But a regular direct insert statement  works like a charm.