AIX defaults to ksh88. My first job out of college a long time ago was to maintain and enhance AIX's various installers, which involves some 100KLOC (!!) of ksh scripts. I was one of the first people on the team to move newer projects to Perl- this was early 00's and somewhat fashionable back then, and the only other scripting language supported on AIX at the time. In retrospect, it's possible everything might have been better off left as ksh :-)