Before you get too much code written using the old XML::Parser API, I'd recommend you switch to SAX. The SAX API is very similar (so it's easy to switch) but it's object oriented, so you can store anything you like (eg: the filehandle) in your object and it will be available whenever one of your handler methods is called.

The Perl XML FAQ links to a number of articles by Kip Hampton including this one on XML::SAX::Machines.

If you install XML::SAX, be sure to install XML::SAX::Expat too - it's faster than the pure Perl parser in the SAX distribution.

PS: PerlMonks is a better place for this type of question.