The recursive example is quite a simple and good one. If you don't understand it then you should read up on the topic of recursion. It's quite a widely used concept in programming and it can make things that would otherwise be very hard to program, quite easy and concise. Developing a good understanding of what recursion is, how/why it works, and when it's a good idea to use it will help you not just solve your current problem, but become a better programmer.

Wikipedia has a reasonably good article on recursion.

If this helps, here's a slightly rewritten version of your recursive function:

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);
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