Besides Perl5, I consider C and C++ as the only other languages which seem eternal, and therefore worth learning.
It has been said that Perl is just Lisp with syntactic sugar, which surely has some points, at the language level. At the implementation level, I would say that perl is just a baroque FORTH engine (implemented in C, which is funny). And therefore, I'd include both Lisp and FORTH (which is still alive and kicking) in the group of -ahem- eternal languages.
Well, thinking again... maybe the triade is just C, Lisp and FORTH, perl being a successful attempt to bind all three into one funny language...
perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'