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<book:bookResource>
<book:book>
<book:meta coverImage="/cover.gif" doi="10.1088" publicationDate="2016
+-12-01" issn="1111-222X" collectionId="0001">
...
</book:meta>
</book:bookResource>
</book:book>

use XML::LibXML;

...
        print $book_node->findvalue($book_element);
        }
}