Volume[@VolumeCategory="L"]
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/Volume[@VolumeCategory="L"]
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They fail to find a match since there's no element named Volume at the root of the document.

/root/Volume[@VolumeCategory="L"]
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would find the node in question, and so would its relative equivalent

root/Volume[@VolumeCategory="L"]
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It's just like directories.

$ cd /

$ ls -d ikegami
ls: cannot access ikegami: No such file or directory

$ ls -d /ikegami
ls: cannot access /ikegami: No such file or directory

$ ls -d /home/ikegami
/home/ikegami

$ ls -d home/ikegami
home/ikegami
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Thanks for the reply.

I did muff my example as I had been using getElementsByTagName which spared me the need to qualify the full path if may say.

Should I understand that that methods does not support attribute?

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.

//title[@lang='en']

Selects all the title elements that have an attribute named lang with 
+a value of 'en'
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my @vol = $doc->findnodes(q {//Volume[@VolumeCategory="L"]});
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use strict;
use warnings;
use XML::TreeBuilder;

my $doc = XML::TreeBuilder->new();
$doc->parse_file("minimal.xml");

my @vol = $doc->look_down(_tag => 'Volume', VolumeCategory => 'L');
# also tried various quoting scheme...
 my $tmp = scalar(@vol);
 print "Number of entries: $tmp \n"; # 1
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..and this a XML::Twig solution.. ;=)


use warnings;
use strict;
use XML::Twig;

my $xml=<<'XML';
<root>
<ProductKey>99</ProductKey>
<Volume VolumeCategory="L" MeasurementCategory="Real">0.063</Volume>
<Volume VolumeCategory="L" MeasurementCategory="Real">0.098989</Volume
+>
<Volume VolumeCategory="cuft" MeasurementCategory="Real">2.2</Volume>
</root>

XML

my @vol_cat_L;
my $twig= new XML::Twig(
                          twig_handlers => { '/root/Volume[@VolumeCate
+gory="L"]' => sub {push @vol_cat_L, $_},
                          },
);

$twig->parse( $xml);
print "Number of L volume category: ",scalar @vol_cat_L,"\n";

##OUTPUT (note the line in example data added by me):
Number of L volume category: 2
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HtH
L*

There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

my $xquery =  '//metadata/audience/@name';# find the first <audience n
+ame="xyz">    
my $xquery2 = "//othermeta[\@name = 'plugin.id']/\@content";# find an 
+othermeta with a name = plugin.id and @content 
my $xquery3 = '//@xml:lang'; # find first @xml:lang

my $xparser = XML::LibXML->new();  # create new parser object
     
eval { $xmldoc = $xparser->parse_file($file) };  # trap parsing errors
+ that can't be suppressed normally, like &nbsp; entities
if ($@){
    print "Problems!  \n The error(s):\n$@\n";  
}
             
if ($pluginname = $xmldoc->findvalue( $xquery2 )){    # returns value 
+of plugin name
    print " Plugin name found: $pluginname\n";
}
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/*[ local-name() = "root"
    and position() = 1
  ]

/*[ local-name() = "Volume"
    and @VolumeCategory = "cuft"
    and @MeasurementCategory = "Real"
    and contains(string(), "2.2")
  ]
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I'll have to give that a try... Thanks.