Are both your installed LibXML and perl 64bit builds? One of the problems on AIX is that you cannot see from the presence of a lib if it is a 32bit lib or a 64bit lib.

If e.g. you are using a 64bit perl build which once worked with a libxml2.a that contained a 64bit set of objects, and then "upgraded" LibXML to a newer version that only shipped a 32bit set of objects (or you installed it by hand and thereby unknowingly removed the 64bit set of objects), then you will not be able to get this to work.

$ file libxml2.a
libxml2.a: archive (big format)
$ ar -X32 -tv libxml2.a
$ ar -X64 -tv libxml2.a
rwxrwxrwx   203/200   2822289 May 12 13:23 2012 libxml2.so.2
$
[download]

In this example, my libxml2.a *only* contains 64bit objects.

You can check if everything would theoretically be able to load:

$ perl -MV=XML::LibXML
XML::LibXML
        /pro/lib/perl5/site_perl/5.14.2/aix-64all/XML/LibXML.pm: 2.010
+3
$ ldd /pro/lib/perl5/site_perl/5.14.2/aix-64all/auto/XML/LibXML/LibXML
+.so
/pro/lib/perl5/site_perl/5.14.2/aix-64all/auto/XML/LibXML/LibXML.so ne
+eds:
         /usr/lib/libc.a(shr_64.o)
         /pro/local/lib/libxml2.a(libxml2.so.2)
         /unix
         /usr/lib/libcrypt.a(shr_64.o)
         /usr/lib/libiconv.a(shr4_64.o)
         /usr/lib/libpthread.a(shr_xpg5_64.o)
$
[download]

Note the added /auto/. If ldd does not report any problems, chances are you will be able to use XML::LibXML.