my $string = q|
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" xmlns:junos
+="http://xml.juniper.net/junos/12.3R9/junos">
<vlan-information junos:style="terse">
<vlan>
<vlan-name>Vlan</vlan-name>
</vlan>
</vlan-information>
</rpc-reply>|;

my $jparser = XML::LibXML->new;
my $doc = $jparser->parse_string($string);

my @vlans = $doc->findnodes("//vlan-information/vlan");
print "found ", scalar @vlans, " vlans\n";

print "The root element's namespace is: ",
    $doc->documentElement->namespaceURI, "\n";

# Give that namespace a prefix so that we can reference it in XPath
$doc->documentElement->setNamespaceDeclPrefix("", "x");

@vlans = $doc->findnodes("//x:vlan-information/x:vlan");
print "found ", scalar @vlans, " vlans\n";

foreach my $vlan (@vlans){
    my @parameters = $vlan->findnodes("./x:vlan-name");
    print "found ", scalar @parameters, " parameters\n";
}
[download]

found 0 vlans
The root element's namespace is: urn:ietf:params:xml:ns:netconf:base:1
+.0
found 1 vlans
found 1 parameters
[download]

So there you have it :)

/*[ local-name() = "rpc-reply"
  ]
/*[ local-name() = "vlan-information"
  ]

/*[ local-name() = "vlan-terse"
  ]
[download]

Create a context and assign prefixes to the namespaces. The prefixes you choose need not match those used in the document, if any.

use XML::LibXML::XPathContext qw( );

my $xpc = XML::LibXML::XPathContext->new();
$xpc->registerNs('nc', 'urn:ietf:params:xml:ns:netconf:base:1.0');
$xpc->registerNs('j', 'http://xml.juniper.net/junos/12.3R9/junos');
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You don't appear to be using the second namespace in your queries. If it's the case, you don't need to register a prefix for it.

Then change $node->find*($xpath) to $xpc->find*($xpath, $node), and use the defined prefixes to specify the namespace of the elements you want to locate.

$xpc->findnodes(
   "/nc:rpc-reply/nc:vlan-information/nc:vlan", $doc)

$xpc->findnodes(
   "nc:vlan-name|nc:vlan-tag|.//nc:vlan-member-interface", $vlan)
[download]

(Removed needless instances of "./".)

Thanks everyone for your insightful (and entertaining?) comments - yes, this is the OP, but being new I may have contravened best practice for changing my username (my apologies to the gods, and other fellow monks). The suggested solutions were all very helpful in understanding the nuances of namespaces (no namespace vs. default namespace and so on) and the multiple ways to come at this sort of problem. I have a further question now with regard to attributes. Whats the best method to pull out the value of a given attribute (like junos:style="terse" in the vlan-information element), especially when the attribute is in a different namespace as the containing element. Using the same XML snippet as in the original post:

my $dom = XML::LibXML->load_xml(
    string  => $jnx->{'server_response'},
    # parser options ...
);

my $xpc = XML::LibXML::XPathContext->new();
$xpc->registerNs('nc', 'urn:ietf:params:xml:ns:netconf:base:1.0');
$xpc->registerNs('j', 'http://xml.juniper.net/junos/12.3R9/junos');

my @nodes = $xpc->findnodes( "/nc:rpc-reply/nc:vlan-information", $dom
+);
foreach my $node (@nodes){
    print "\nAttempt 1\n";
    my $attr_style = $node->findvalue('/@style');
    $attr_style ? print "Got Attr _style_: $attr_style\n" : print "Cou
+ldn't find Attr _style_\n";

    print "\nAttempt 2\n";
    my $test1 = $node->hasAttributes();
    print " Yes I have attribute(s)\n" if $test1;

    print "\nAttempt 3\n";
    my @attrs = $node->attributes('/style');
    foreach my $attr (@attrs){
        print "here's an attribute: $attr->to_literal\n";
    }   

    print "\nAttempt 4\n";
    my $attrnode = $node->getAttributeNodeNS( 'http://xml.juniper.net/
+junos/12.3R9/junos', 'style');
    print "here's an attribute: $attrnode\n";

}
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My attempts above (the last two at least) yield a result but its not ideal... I want to say for attribute "style" in namespace "junos", what is the value? Thanks!

#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd /;
use XML::LibXML 1.70; ## for load_html/load_xml/location

sub XML::LibXML::Node::F {
    my $self   = shift;
    my $xpath  = shift;
    my %prefix = @_;
    our $XPATHCONTEXT;
    $XPATHCONTEXT ||= XML::LibXML::XPathContext->new();
    while( my( $p, $u ) = each %prefix ){
        $XPATHCONTEXT->registerNs( $p, $u );
    }
    $XPATHCONTEXT->findnodes( $xpath, $self );
}


my $doc = XML::LibXML->new()->parse_string(q{<rpc-reply 
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" 
xmlns:junos="http://xml.juniper.net/junos/12.3R9/junos">
<vlan-information junos:style="terse">
<vlan-terse/>
<vlan>
...
</vlan>
</vlan-information>
</rpc-reply>
});

## "registration" find a node that don't exist
$doc->F( "/NOTExIST", 
    'nc', 'urn:ietf:params:xml:ns:netconf:base:1.0',
    'j', 'http://xml.juniper.net/junos/12.3R9/junos',
);

for my $vlaninfo( $doc->F( "/nc:rpc-reply/nc:vlan-information", ) ){
    my( $style ) = $vlaninfo->F('./@j:style');
    print "$style\n";
    dd( { %$vlaninfo });
}
__END__
 junos:style="terse"
{ "{http://xml.juniper.net/junos/12.3R9/junos}style" => "terse" }
[download]