select f1 from t1 where t1 in (:p1)

SQL> var p1 varchar2(200)
SQL> exec :p1:='509,10006,10018,2102956,1103456';

PL/SQL procedure successfully completed.

SQL> ed
Wrote file afiedt.buf

  1  with t as (
  2  select 509 n   from dual union all
  3  select 956 n   from dual union all
  4  select 1103456 from dual )
  5  select * from t
  6* where instr(','||:p1||',',','||n||',')>0
SQL> /

         N
----------
       509
   1103456

SQL> select :p1 p1 from dual;

P1
-----------------------------------
509,10006,10018,2102956,1103456

CREATE OR REPLACE TYPE "CHARLIST" as table of varchar2(4000)

select column_value from table(charlist(:p1));