get_node is implemented much like this:

sub get_node {
   my ($self, $pos) = @_;
   $self->[$pos - 1];
}
[download]

So $nodelist->get_node(0) is the same as $arrayref->[-1].

I wouldn't rely on it if I were you though. It's undocumented behaviour that is merely an accident of the internal implementation of XML::LibXML::NodeList, and could theoretically change in the future.