Shooting from the hip, you need to change your data structure from

push(@{$ref{'node'}},{
  'id'   => 'net_1.2.3.0',
  'desc' => 'network',
});
[download]

to

push(@{$ref{'node'}},{
  'id'   => 'net_1.2.3.0',
  'desc' => ['network']
,
});
[download]

Or something similar.

An interesting note from the XML::Simple documentation: "The use of this module in new code is discouraged. Other modules are available which provide more straightforward and consistent interfaces. In particular, XML::LibXML is highly recommended." While I don't necessarily agree with the recommendation for XML::LibXML myself, I would take this under advisement and look for something else. I can highly recommend XML::Twig for ease of use and power.