%hash only has one key (how useless!), so

@HoH{keys %hash} = values %hash
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is the same as

$HoH{$unit_num} = $hash{$unit_num};
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As you can clearly see now, you completely replace all existing data for $unit_num every time you find a new record for $unit_num.

Change

  %hash = (
             $unit_num => {
                $event_nm => [ $event_date ],
   },
  );

  @HoH{keys %hash} = values %hash
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to

  push @{ $HoH{$unit_num}{$event_nm} }, $event_date
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Thanks to autovivification, the above is short for

  $HoH{$unit_num}            ||= {};
  $HoH{$unit_num}{$event_nm} ||= [];
  push @{ $HoH{$unit_num}{$event_nm} }, $event_date;
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Tip: Don't pass hashes and arrays to Dumper. Pass references to them:

print Dumper( \%HoH );
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