If you want to search the whole document, you can use

/descendant::Volume[@VolumeCategory="L"]
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which can be abbreviated to

//Volume[@VolumeCategory="L"]
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Should I understand that that methods does not support attribute?

If you're asking if getElementsByTagName can filter the returned nodes to retain only those with a specific attribute value, then the answer is no. The only argument it takes is a tag name. You could do

my @matching =
   grep {
      my $vc = $_->getAttribute('VolumeCategory');
      defined($vc) && $vc eq "L"
   }
      $doc->getElementsByTagName('Volume');
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but aforementioned

my @matching = $doc->findnodes('//Volume[@VolumeCategory="L"]');
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is much simpler.