# specify twig_handlers (see the XML::Twig documentation)
twig_handlers => 
   { # create a new hash-ref
       map { # transform a list into a new list 
             # create a name / subroutine pair
             $_ => sub {
                # when the subroutine is called
                # get the element name from $_->name()
                # and increase it's count entry in %found
                #
                # note: $_ is set by XML::Twig when the sub is called
                $found{$_->name()}++ 
             }; 
       } @nodes # for each element in @nodes
   }