What is wrong with registerNs()?

my $dom = $parser->parse_file("book.xml");
my $root = $dom->documentElement();
my $xpc = XML::LibXML::XPathContext->new($root);

$xpc->registerNs(xlink => "http://www.w3.org/1999/xlink") or die "Coul
+dn't register Namespace - $!";
[download]

The xml file is :

<?xml version="1.0" encoding="UTF-8"?>

<book:bookResource xmlns:xlink="http://www.w3.org/1999/xlink">
<book:book>
<book:chapter id="bk111111ch1" type="CHAPTER">
<book:locator xlink:href="/book/isbn/979-0-1234-1111-1/book-part/chapt
+er/bk111111ch1?releaseStatus=UNRELEASED" xlink:title="Photonic crysta
+l light-emitting sources" xlink:type="locator" xmlns:xlink="http://ww
+w.w3.org/1999/xlink"/>
<book:locator xlink:href="/book/isbn/979-0-1234-1111-1/book-part/chapt
+er/bk111111ch1?releaseStatus=UNRELEASED&amp;format=pdf" xlink:title="
+Photonic crystal light-emitting sources" xlink:type="locator" xmlns:x
+link="http://www.w3.org/1999/xlink"/>
<book:locator xlink:href="/book/isbn/979-0-1234-1111-1/book-part/chapt
+er/bk111111ch1?releaseStatus=UNRELEASED&amp;format=epub" xlink:title=
+"Photonic crystal light-emitting sources" xlink:type="locator" xmlns:
+xlink="http://www.w3.org/1999/xlink"/>
<book:meta doi="10.1088/bk111111ch1" firstPage="1-1" lastPage="1-118">
<book:author givenName="Roger" surname="Ebert"><book:affiliation xlink
+:href="bk111111ch1af1" xlink:type="locator" xmlns:xlink="http://www.w
+3.org/1999/xlink"/></book:author>
<book:affiliation name="Cavendish Laboratory, University of Cambridge"
+><book:locator xlink:href="bk111111ch1af1" xlink:type="locator" xmlns
+:xlink="http://www.w3.org/1999/xlink"/></book:affiliation>
</book:meta>
</book:chapter>
</book:book>
</book:bookResource>
[download]