for my $item($doc) {

This almost certainly isn't doing what you think it is. $doc isn't a list, it's a reference to an XML::Lib document. So effectively, all you're doing there is aliasing $item to $doc. Of course you completely ignore $item within the block, so it's not doing any actual damage. But your loop is only ever being run once.

I think you actually want something more like this (but please note this is untested):

for my $item ($doc->findnodes('//channel/item')) {
  my $date = $item->findvalue('pubDate');
  my $desc = $item->findvalue('description');
  $seen{$date} = localtime;
  next if $SAW{$date};
  push @item_output, $desc;
}
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But it's also worth pointing out that if you're dealing with RSS documents, then XML::RSS is far easier than using a generic XML tool like XML::LibXML.