Shouldn't you be looping on the keys of %date_slots instead of %{$HoH{$customer}}?

foreach $customer (keys %HoH) {
       print "$customer\n";
       foreach $date (keys %date_slots ) {
               print "\t\t$date";
               foreach my $data (keys %{$cal{$customer}{$date}} ) {
                       if (defined $data) {
                         print " = $data\n";
                       } else {
                         print " = no data\n";
                       }
               }
       }
}
[download]

-- Dan