Looking at the doc for XML::Twig I would say not. Like the example I gave, XML::Parser is used, because it is SAX-based and therefore very kind to memory - only the parts you specify are in memory once the parse is complete.

From the doco for XML::Twig

This module provides a way to process XML documents. It is build on top of XML::Parser.

It allows minimal resource (CPU and memory) usage by building the tree only for the parts of the documents that need actual processing, ...

As for XML::XPath, it doesn't mention it explicitly, but a quick surf through the source for its internal class XML::XPath::XMLParser, shows that it too uses XML::Parser. And it declares handlers for the events of XML::Parser. But it seems to build up an internal tree (of arrayrefs, and the author states the reason for this is speed). I see lots of code like $self->{current}->appendChild($node, 1);
so perhaps it is does build an internal image first.

Why not test with a huge XML doc and watch the memory footprint via ps or top