If you register the namespace using XML::LibXML::XPathContext you can then use XPaths to select nodes as follows:

#!/usr/bin/perl

use strict;
use warnings;

use XML::LibXML;
use XML::LibXML::XPathContext;

# load the XML doc
my $p = XML::LibXML->new;
my $xml_file = do { local $/; <DATA> };
my $dom = $p->parse_string( $xml_file );

# register the namespace
my $xc = XML::LibXML::XPathContext->new( $dom );
$xc->registerNs('ns', 'http://www.mydomain.com');

# select using XPath
my @nodes = $xc->findnodes( '/ns:info/ns:city');
print $_->toString for @nodes;

__DATA__
<?xml version="1.0"?>

<info
xmlns="http://www.mydomain.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.mydomain.com infoschema.xsd">

<name>john</name>
<city>baltimore</city>
<zip>21205</zip>
</info>
[download]