for my $customer ( keys %$cust ) {
    my $total_usage = 0;
    ...
        ...
            for my $direction qw(OUT IN) {
                my $usage = $bill_data ->{$customer}->{$device}->{$port}->{$direction};
                $total_usage += $usage;
                print CUST_OUT "$lastmonth_period $device $short_port $direction$usage$/";
            }
    
    ... print out $total_usage
    close(CUST_OUT);
}