Is there a function to split a string in PL/SQL?

If APEX_UTIL is not available, you have a solution using REGEXP_SUBSTR().

Inspired from http://nuijten.blogspot.fr/2009/07/splitting-comma-delimited-string-regexp.html :

DECLARE
  I INTEGER;
  TYPE T_ARRAY_OF_VARCHAR IS TABLE OF VARCHAR2(2000) INDEX BY BINARY_INTEGER;
  MY_ARRAY T_ARRAY_OF_VARCHAR;
  MY_STRING VARCHAR2(2000) := '123,456,abc,def';
BEGIN
  FOR CURRENT_ROW IN (
    with test as    
      (select MY_STRING from dual)
      select regexp_substr(MY_STRING, '[^,]+', 1, rownum) SPLIT
      from test
      connect by level <= length (regexp_replace(MY_STRING, '[^,]+'))  + 1)
  LOOP
    DBMS_OUTPUT.PUT_LINE(CURRENT_ROW.SPLIT);
    MY_ARRAY(MY_ARRAY.COUNT) := CURRENT_ROW.SPLIT;
  END LOOP;
END;
/

There is apex_util.string_to_table - see my answer to this question.

Also, prior to the existence of the above function, I once posted a solution here on my blog.

Update

In later versions of APEX, apex_util.string_to_table is deprecated, and a similar function apex_string.split is preferred.