Just in case... :)

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