On AIX, I build my own version of perl and install it in a location the system doesn't manage (e.g. /usr/local/perl/), and has no symbolic links to. That way I'm guaranteed that some future upgrade of AIX won't magically replace what I install, even if what I'm using is old. Also, I sometimes need features that IBM decides not to put into their build (e.g. 64-bit integers -- not sure if they do that now though).