Ultimately I think all programming languages, like spoken languages, will turn out to have a limited shelf life. Even the mighty C will eventually fall, or at least evolve into something almost as unrecognizable to today's C programmers as Beowulf is to today's English speakers. (C will just take a lot longer than most other programming languages.)
Perl 5 seems pretty healthy for the time being though.
When Perl 5 does eventually die, I hope that it's because it was eaten by Perl 6. Perl 6 will never be a matter of a simple upgrade - it's an entirely different programming language. If it ever gets itself into a position where it can eat Perl 5, that will be a Good Thing, because it will mean that it can probably also eat Ruby, Python, and most other dynamic and high-level programming languages. If all those programming languages get eaten by Perl 6, that will be a Good Thing for programmers, as Perl 6 (the language) is pretty awesome, even if Perl 6 (the implementations) are not quite there yet.