my $xpc = XML::LibXML::XPathContext->new;
$xpc->registerNs( xsi => 'http://www.w3.org/2001/XMLSchema-instance' );

my $doc = XML::LibXML->new->parse_XXX(...);
my $schema_loc = $xpc->findvalue('/*/@xsi:schemaLocation', $doc)
   or die("...\n");

my ($schema_ns, $schema_url) = split(' ', $schema_loc);
defined($schema_url)
   or die("...\n");