Generate new VARCHAR2 type variable to assign your IN (input) string.
procedure sp_name(
ps_list IN VARCHAR2,
...
other IN's and OUT's
...
)
as
ps_list_copy VARCHAR2 (32000);
begin
ps_list_copy := ps_list;
...
do your works with ps_list_copy
...
...
Exception when others then
....
end sp_name;