How do i get value of an element if the next elememnt has specific value in XML::LibXML using Xpath?

[ravi06]

<book:bookResource>
<book:book>
<book:meta coverImage="/cover.gif" doi="10.1088" publicationDate="2016
+-12-01" issn="1111-222X" collectionId="0001">
<book:title title="Lattice dynamics">
<book:subTitle>Book 17</book:subTitle></book:title>
<book:isbn isbn="979-0-4444-1000-17" type="epub"/>
<book:isbn isbn="979-0-4444-2000-2" type="ppub"/>
</book:meta>
</book:bookResource>
</book:book>

How can I get isbn="979-0-4444-1000-17" for type="epub"? I tried the following code, but of no use. Help me please.

use XML::LibXML;

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

foreach my $book_node ($xpc->findnodes('/book:bookResource/book:book/b
+ook:meta'))
{
    foreach $book_element ('./*/@isbn[@type="epub"]')
    {
        print $book_node->findvalue($book_element);
        }
}