First, I have to agree that a killer app for a language is a fuzzy concept at best. Email is a killer corporate app, Office is a killer Windows app (or suite), the Web is the killer app that brought the Internet out of the geek closet and into everyone's conciousness. But those are all specific contexts. In the Perl context, though? CPAN probably is the killer app. Perl regexes were for a long time, such that if you thought about text munging, you automatically thought of Perl. What did you have in mind when asking the question?
But since this came up, I thought I'd share some thoughts I've had recently. What's to become of Perl? It no longer shines on a resume; instead it appears on job descriptions as part of "Python/Perl/Shell" or just "scripting experience". The old cowbell of "ugly code" is heard more and more, with people who don't know what they're talking about mentioning Python or Ruby to show how Perl is no longer relevant.
And now, with Perl 6 on the horizon, everyone who considered him/herself a Perl hacker will be crippled while learning the new language; CPAN will be suspect as you work out whether a module you like will work with Perl 6; every book written about Perl will be out of date, referring to the "old" syntax. Is Larry/Perl forging something new, using the same insight that synthesized existing technology into something uniquely useful? Or are we playing catch-up?
I'd rather Python/Ruby/Java developers feel they had to learn Perl to stay relevant than the other way around.