I see a completely unrelated problem with your code:

  foreach my $e (@calendar) {  $union{$e} = 1  }
  
  foreach $e (@Calendararray)  {
      if ($union{$e})  {  $intersect{$e} = 1  }
      $union{$e} = 1;
  }
  
  @intersect = keys %intersect;
  @union = keys %union;
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What will happen when you have these values (no real values, just to show you what I mean)

@calendar= qw~ 1 ~; 
@Calendararray=qw~ 2 2 ~;
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You will end up with an @intersection containing "2".