Probably this is a long shot...

I used to suffer a similar behaviour, when I used XML::Parser::Expat directly, thus without going through XML::Parser, if it died without me having a chance to do some cleanup. The problem was indeed that this parser uses circular references internally, and you have to call its release method in order to break these chains. Actually, I suspect one of the reasons for the existence of XML::Parser as a wrapper, is precisely to do this cleanup for you, in case of an error.

So.... if you have circular references that aren't broken before the final curtain falls, apparently this is what you can expect to happen.

Are you sure you did indeed break all circular references, if present, by just undeffing the variables? I doubt it.