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');

[download]

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};

[download]

In xsh, the whole program can be shortened to 

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

[download]

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: Replace xml first id by choroba
in thread Replace xml first id by michael99

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