You have this:

# -- hash to store found exception dates
my %fnd_excep = ();
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This creates an empty hash (i.e. a list of key-value pairs). However, at no point in the subsequent code, is any data actually stored into that hash.

The hash is then looped through to generate the HTML, but as there's nothing in the hash, there's nothing to loop through.

You probably want to be populating the hash inside this loop:

# -- loop thru the exception data from Voyager table
foreach my $row ( @$arrayref_exc_dates ) {
   
   my ( $exception_date,$exception_openhour,$exception_closehour ) = @
+$row;
   if ( not defined $exception_openhour ) { $exception_openhour = "clo
+sed" }
   if ( not defined $exception_closehour ) { $exception_closehour = "c
+losed" }

   # Maybe try something like this!!
   $fnd_excep{$exception_date} = {
     open   => $exception_openhour,
     close  => $exception_closehour,
     };
}
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