sub do_something_recursively
{
   my ($element) = @_;
   
   # Here we will do something just to $element
   # without worrying about recursion at all.
   $element->set_class("processed");
   
   # Now we loop through each child element
   foreach my $child ($element->content_list)
   {
      # Skip text nodes
      next unless ref $child;

      # And we call *this function* on the child
      do_something_recursively($child);
   }
}

do_something_recursively($root_element);