You could try XML::Validator::Schema for a start. But I've never used it, so searching for "schema" on CPAN might yield better modules.

You can do it with XML::LibXML. Make sure that you have the latest libxml2 library installed (http://xmlsoft.org/), as it has improved schema validation. For instance, Redhat Linux came with an older version of libxml2, so I installed the latest version from source. The default location is different to the standard Linux build, so I reinstalled XML::LibXML and XML::LibXML::Common to point to it.

use XML::LibXML;

my $schema_file = 'po.xsd';
my $document    = 'po.xml';

my $schema = XML::LibXML::Schema->new(location => $schema_file);

my $parser = XML::LibXML->new;
my $doc    = $parser->parse_file($document);

eval { $schema->validate($doc) };
die $@ if $@;

print "$document validated successfully\n";

For some reason, this is not documented in the POD (which is why almost no one knows about XML:LibXML's schema validation powers!), but the XML::LibXML source comes with the documentation in docbook format plus a utility to generate the POD from that.