Disclaimer: I've never used XML::XPath and don't even have it installed. I'm writing this from a quick glance at the documentation and my experience working with XML::LibXML. You should get the idea.

for my $available ( $xp->findnodes( '//Available' ) ) {
    my $qty  = $available->findvalue( 'Qty' );
    my $part = $available->findvalue( 'Part' );
    # ...
}
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The important part is to realize that the XPath expressions passed to $available->findvalue() will be evaluated relative to the node $available represents.