sub setup_file_name_generator {
    # Generate file names of the form
    #   "year-month-day-hour-pid-count".
    #
    # The year-month-day-hour-pid portion
    # is cached and will expire on the hour.

    my $time = time;
    my @time = localtime($time);
    my $base = sprintf('%4d-%02d-%02d-%02d-%d-',
               $time[5]+1900, $time[4]+1, $time[3],
               $time[2], $$);
    my $count = 0;
    my $expires = $time + 60 * (60 - $time[1]);

    *generate_file_name = sub {
        $time = time;
        if ($time >= $expires) {
            setup_file_name_generator();
            return generate_file_name();
        }
        else {
            my $name;
            do {
                ++$count;
                $name = $base . $count;
            } while (-e $name);
            return $name;
        }
    }
}

setup_file_name_generator();

my $name = generate_file_name();