use strict;
use XML::LibXML;

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

my @vol = $doc->findnodes(q {Volume[@VolumeCategory="L"]});
# also tried various quoting scheme...
my $tmp = scalar(@vol);
print "Number of entries: $tmp \n"; # 0 

##</code><code>##

<root>
<ProductKey>99</ProductKey>
<Volume VolumeCategory="L" MeasurementCategory="Real">0.063</Volume>
<Volume VolumeCategory="cuft" MeasurementCategory="Real">2.2</Volume>
</root>