The LibXML documentation says "This module is an interface to the gnome libxml2 DOM parser (no SAX parser support yet), and the DOM tree." So is it still acceptable in your opinion?
It doesn't matter whether anyone else finds this restriction acceptable or not. You need to determine whether this may cause problems for you or not and compare those problems to the benefits you gain from using this code. It depends on your situation.
Any modern Unix should run libxml2 and some come with it installed or as part of their package system. If you run Windows, PPMs exist for ActivePerl. Windows binaries of libxml2 exist, if you use other versions of Perl on Windows.
So, code that uses XML::LibXML should run on Unix, including Mac OS X, and Windows. If you need to port your code to other platforms, investigate each platform and see if XML::LibXML runs on it.