for my $customer ( keys %$cust ) {

    # If you want to write to a file named with the customer ID
    # my $output = "./$customer.out";

    # If you want to write to a file named with the customer name
    my $output = "./$cust->{$customer}->{name}.txt";
    open (CUST_OUT, ">$output") || die "Can't write to '$output': $!\n";
	
    print CUST_OUT "Data Transfer Report for $cust->{$customer}->{name}\n" .
        "Period                         Device_Code      Port        Traf_Dir Data_Usage\n";

    for my $device ( sort by_name keys %{$bill_data->{$customer}} ) {

        for my $port ( sort keys %{$bill_data->{$customer}->{$device}} ) {

            (my $short_port = $port) =~ s/^FastEthernet/Fa/g;

            for my $direction qw(OUT IN) {
                print CUST_OUT "$lastmonth_period $device $short_port $direction"  .
                    $bill_data ->{$customer}->{$device}->{$port}->{$direction} . $/;

	 } 
         
      }       

    }

    close(CUST_OUT);

}