Since you didn't provide any input data, it is impossible for anyone to re-create your problem. Thus, I can only offer generic advice.

Firstly, use strict and warnings. Perhaps some of your variables are not scoped properly.

Secondly, sprinkle print statements liberally throughout your code to make sure your variables contain what you expect them to contain. See also Basic debugging checklist.

At the state of the art, I can't change my parsing module

It would be a good idea to have a plan in place to retire XML::Parser in favor of a more user-friendly (in my opinion) and better supported parser, such as XML::Twig.