expat can be a pain.
It does not compile on Tru64 (Digital^WCompaq^WHP Unix on alpha), so effectively you cannot process XML with perl on Tru64.
Update: On Tru64 gcc is not recommended, the vendor delivers a C-compiler that is better than gcc. Perl is almost invariably compiled with the vendor suplied compiler (which is a bit pickier than gcc in adhering to standards and error checking). Thus some^Wtoo many OS projects are unavailable on Tru64 - but this is getting a bit OT.
XML::Parser 2.29 and earlier were supplied with a version of expat that compiles almost as many places as Perl, I have hung on to that version ;-)