with data as (
select 1 as t from dual union all
select 2 as t from dual union all
select 3 as t from dual union all
select 4 as t from dual union all
select 5 as t from dual 
)
select * from data where t in (2,3)

CREATE TYPE numbers_nestab_ty AS TABLE OF NUMBER(5);
/

CREATE OR REPLACE FUNCTION myfunction(param_values IN numbers_nestab_ty)
RETURN ... PIPELINED AS ...
    l_indx PLS_INTEGER := 0;
BEGIN
    l_indx := param_values.FIRST;
    WHILE l_indx IS NOT NULL
    LOOP
        ...
        l_indx := param_values.NEXT(l_indx);
    END LOOP;
    ...
    RETURN ...
END myfunction;
/

SQL> var mylist varchar2(100)
SQL> exec :mylist := '5,11,13,22,23,31,44,45'
SQL> with list as (
  2    select substr(:mylist,
  3                  instr(','||:mylist||',', ',', 1, rn),
  4                  instr(','||:mylist||',', ',', 1, rn+1)
  5                  - instr(','||:mylist||',', ',', 1, rn) - 1) value
  6    from (select rownum rn from dual 
  7          connect by level 
  8                       <= length(:mylist)-length(replace(:mylist,',',''))+1)
  9    )
 10  select id, valeur
 11  from t
 12  where id IN ( select value from list )
 13  order by id
 14  /
        ID USERNAME
---------- ------------------------------
         5 SYSTEM
        11 OUTLN
        22 MICHEL
        23 OPS$MCADOT101205
        31 SCOTT

5 rows selected.