create or replace type table_of_varchar is table of varchar2( 255 );

declare
    l_headers table_of_varchar;
begin
    select a
    bulk collect into l_headers
    from emp;

    return l_headers;
end;

declare
    l_headers table_of_varchar;
begin
    select 'value' as a
    bulk collect into l_headers
    from dual;

    return l_headers;
end;