how are you validating your XML?

I am very new to XML and I am wondering, how do I validate my XML.

Let's say I have an XML document and an XML Schema. What program would I use to validate the XML against the Schema?

I would like to be able to call a function from perl, pass it the XML document with the schema, and receive the results back.

Does such a thing exist?

I found a simple way to do validation:

http://www.ascc.net/xml/resource/schematron/

I'm using the perl version found in cpan.