This is not a modified Xpath but shows how you can access the child nodes:

use XML::LibXML;

my $string = q|
<Root>
  <Parent>
    <child1>Child 1</child1>
    <child2>Child 2</child2>
    <child3>Child 3</child3>
  </Parent>
</Root>|;

my $doc = XML::LibXML->load_xml(string => $string);
my @nodes = $doc->findnodes('//Parent');
for my $node (@nodes) {
    my @childnodes = $node->childNodes or next;
    for my $cnode (@childnodes) {
        if ($cnode->nodeName =~ m/^child/) {
            print 'name: '. $cnode->nodeName . ', ';
            print 'content: '. $cnode->textContent ."\n";
        }
    }
}

# Output:
# name: child1, content: Child 1
# name: child2, content: Child 2
# name: child3, content: Child 3
[download]

See XML::LibXML::Node