With hashes, you can easily check to see if a key exists using the exists function. As the key does not exist in HoH hash, it should therefore be a simple matter to print "no data": (untested code)

foreach my $quarter (keys %date_slots) {
  if (exists $HoH{$customer}{$quarter} {
    # print data
  } else {
    print " = no data";
  }
}
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Is only a quick reply .. hope it helps a little.
--Foxcub