AND EMPLOYEE.EMPLOYEE_ID IN (var_employee_id)

create table in_test (col_1  varchar2(10));

insert into in_test values ('a');
insert into in_test values ('c');
insert into in_test values ('f');
insert into in_test values ('g');

DECLARE
  v_string   varchar2(30);
BEGIN
  dbms_output.put_line('Test 1');
  FOR rec in (SELECT col_1
              FROM   in_test
              WHERE  col_1 in ('a','b','c','d')) LOOP

    dbms_output.put_line('Test 1: Found record '||rec.col_1);
  END LOOP;
  
  v_string := '''a'',''b'',''c'',''d''';

  dbms_output.put_line('Test 2');
  dbms_output.put_line('v_string is '||v_string);
  
  FOR rec in (SELECT col_1
              FROM   in_test
              WHERE  col_1 in (v_string)) LOOP

    dbms_output.put_line('Test 2: Found record '||rec.col_1);
  END LOOP;  
END;
/

Test 1
Test 1: Found record a
Test 1: Found record c
Test 2
v_string is 'a','b','c','d'

AND EMPLOYEE.EMPLOYEE_ID IN ('ww,rr,tt')

AND EMPLOYEE.EMPLOYEE_ID IN (var_employee_id)

SQL> create or replace function err_test (p_in in varchar2) return varchar2 as
  2  begin
  3    raise_application_error(-20001,'Fish!');
  4  end;
  5  /

Function created.

SQL> 
SQL> 
SQL> DECLARE
  2    c   sys_refcursor;
  3    v1  varchar2(40);
  4    v2  varchar2(40);
  5  begin
  6    open c for 'select col_1,err_test(col_1) from in_test';
  7    fetch c into v1,v2;
  8    close c;
  9  end;
 10  /
DECLARE
*
ERROR at line 1:
ORA-20001: Fish!
ORA-06512: at "DEV.ERR_TEST", line 3
ORA-06512: at line 7