All of those UNIXen are going the way of the buffalo as the world moves to the web. I haven't seen HP-UX or AIX system in a decade or more. Admittedly, I haven't done contract work in about eight years, which is where I previously saw those beasts (and others; for a short time I specialized in migrating businesses from SCO UNIX to Linux and made a small fortune doing it), but our software gets installed in all sorts of places and we used to hear about those systems pretty regularly...but the mentions of everything other than Solaris on our mailing lists and forums and IRC has slowed to practically nothing, and Solaris is in a slow spiral downward.
I use Perl daily. Love it. But, the argument that it's available while others are not isn't really valid anymore except in a few niches. Linux is pervasive, and Python and Ruby are pervasive on Linux systems. In fact, Python is more likely to be available on the two biggest distros because many of the administrative tools provided by the distro are written in Python. RHEL/CentOS/Fedora effectively have standardized on Python as the language of choice for admin tools.
I use Perl daily. Love it. But, the argument that it's available while others are not isn't really valid anymore except in a few niches. Linux is pervasive, and Python and Ruby are pervasive on Linux systems. In fact, Python is more likely to be available on the two biggest distros because many of the administrative tools provided by the distro are written in Python. RHEL/CentOS/Fedora effectively have standardized on Python as the language of choice for admin tools.