In a XPath Expression, bare id means "element named id", attributes start with the at-sign (already noted by Corion):

say 'id: ', $object11->findvalue('@id');
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But with XML::LibXML, you can get to an attribute value by a shortcut: you can pretend the element is a hash reference and ask for a key:

say 'id: ', $object11->{id};
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In xsh, the whole program can be shortened to

open test.xml ;
cd header/idset ;
echo (a) ;
echo (b) ;
echo @id ;
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