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>