use warnings;
use strict;
use XML::LibXML;

my $parser = XML::LibXML->new();
my $doc = $parser->parse_file("foo.xml");
$parser->processXIncludes( $doc );

print $doc->toString(1);

__END__

<?xml version="1.0"?>
<foo>
<bar>
  This is bar.xml!
</bar>
</foo>