huge_string_var varchar2(32000);

begin
   fhandle := fopen( my_file );

   while not feof loop
        temp_var := fread( fhandle);
        huge_string_var := huge_string_var || temp_var;

        pos_v := instr( huge_string_var, 'search_string' );
        if pos_v > 0 then
            -- I found the string, so do something
            -- next line is to continue the search for more matches, but if not necessary just exit
            huge_string_var := substr( huge_string_v, pos_v + length( 'search_string' );
         end if;
    end loop;
end;