Implementing XML Security is relatively complex. You take an XML file to be signed, a template that indicates the structure of the signature, algorithms to compute the signature, your public key, and combine according to XML-Signature Syntax and Processing.

A basic tutorial explaining the process is at An Introduction to XML Digital Signatures

I do not know of a Perl module that will implement this for you, but there is an excellent C library, xmlsec, that will sign, encrypt and canonicalize XML documents for you. It comes complete with a tutorial containing several simple examples.