create or replace TYPE array_collection IS table OF VARCHAR2(50)
--string array is working absolutely fine so dw about that

create or replace type date_array is table of date;
--here i don't if I've defined this array correctly

create or replace procedure convert_arr(
dat_ar in array_collection,perdate_arr out date_array)
as
begin
perdate_arr:=new date_array();
perdate_arr.extend(dat_ar.count);
for i in 1..dat_ar.count loop
perdate_arr(i):=to_date(dat_ar(i), 'yyyy-mm-dd');
end loop;
end convert_arr;
--compiles perfectly

set serveroutput on
declare
--l_dat array_collection;
--l_darray date_array;
l_dat array_collection:=array_collection();
l_darray date_array:=date_array();
begin
l_dat := array_collection('2011-01-01','2011-04-01','2011-05-01');
--l_dat.extend(3);
-- l_dat(1):= to_date(2019-07-08);
-- l_dat(2):= to_date(2019-07-09);
-- l_dat(3):= to_date(2019-06-02);

convert_arr(dat_ar=>l_dat,perdate_arr=>l_darray);
dbms_output.put_line('Number of array:' || l_dat.count);
for i in 1..l_dat.count loop
dbms_output.put_line('Date ' || i || ':' || to_char(l_dat(i),'dd/mm/yyyy'));
end loop;
end;

Error report -
ORA-01861: literal does not match format string
ORA-06512: at line 9
01861. 00000 -  "literal does not match format string"
*Cause:    Literals in the input must be the same length as literals in
           the format string (with the exception of leading whitespace).  If the
           "FX" modifier has been toggled on, the literal must match exactly,
           with no extra whitespace.
*Action:   Correct the format string to match the literal.