my @unique_ids = (23456, 23458);
my $xml_source_key = XMLin($xml_file, forcearray => 
   [ qw (thing foo bar bazs) ], keyattr =>[ 'unique_identifier' ]);

foreach my $unique_id (@unique_ids) {
   foreach my $bar_loop ( @{$xml_source_key->{'things'}->{'thing'}->{$unique_id}->{'bars'}->{'bar'} } ) {
      $bar_number = $bar_loop->{'bar_number'};
      $good_info = $bar_loop->{'good'}->{'good_info'};
      
      # Do something with $bar_number, $good_info and a database
   }
}