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if($@) {
  # assign an empty (but defined) string to $bus_url
  $bus_url = '';
...
  # assign tag contents to $bus_url
  $bus_url = $BusUrl->item(0)->getChildAtIndex(0)->getData(); 
}

my $BusUrl = $contact->getElementsByTagName( "BusinessUrl" );

# check if tag is defined and has children
...
  $bus_url = '';
}