Stupid SQL question?

On 9iR2....

Is there a way to use th 'in' clause but comparing to a variable with string value such as

declare
  str varchar2(30) := 'A,B,C';
  i number;
begin
  select count(*)
  into i
  from mytab
  where col in str;
end;

Actually I know this isn't possible but can a similar effect to

..

where col in ('A','B','C')
..

be achieved without supplying a constant to the where clause - for example passing an aray of values for comparison? or do I need to build the SQL dynamically?

ta...

-- 

jeremy