That said, ifyou need to process a document that is too big to fit memory and XML::Twig is too slow for you, my reluctant advice would be to use "bare" XML::Parser. It won't be as easy to use as XML::Twig: basically with XML::Twig you trade some speed (depending on what you do from a factor 3 to... none) for ease-of-use, but it will be easier IMHO than using SAX (albeit not standard), and at this point a LOT faster (see the last test in simple benchmark).

1 GB is going to be slow no matter what. If you have enough memory to load the document entirely in memory, then XML::LibXML will be the fastest option. If you can't load it and you need to use SAX, then performances drop dramatically (as mentioned previously, look at this benchmark). I am not sure the pull interface is available from the Perl module, but that might give you XML::LibXML speed without having to load the whole document (I have never used it myself, so I am not sure if that's even possible). XML::Twig would be a better option than using SAX. And indeed, XML::Parser might be your best bet, even if it's not the easiest module to use.

Beyond that, you get into dangerous territory: format the XML properly, get rid of potential comments, CDATA sections, PIs and entities, and use regexps. I would try the other options before though.