You can access your desired information several ways using XPath. For example:

/Parameter/Value/Item
/Parameter/Value/Item[@Value]
//Item
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These all can get you to the node you desire.
I assume from your Xpath that this is a snippet of the acutal XML file your are parsing. If you want just one of these values, you would want something like the XPath you mention in your post. But if you wanted all the Values in your XML, use the 3rd one I listed.
Check out W3 Schools XPath for more information on XPath.

One of the reasons I prefer XML::LibXML is the XPath support
Try

use XML::LibXML;

#Parse XML
my $template = 'xmldocument.xml';
my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($template);

#Find desired node(s)
my @nodes= $doc->findnodes("//Item");

#Print results
for(@nodes)
{  if($_->hasAttribute("Value")
   {  my $data = $_->getAttributes("Value");
      print "$data\n";
   }
}
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Update:Fixed typo