I guess you're really going to have to ask yourself, is it the language that makes the job interesting for you, or is the satisfaction you get from solving a complex problem?

I primarily use Java on the job, but have a certain amount of freedom to use other solutions when appropriate. Sometimes that includes Perl, and it's a handy way of getting simple and sometimes complex tasks done quickly. I find that it supplements my efforts, and helps me to be more efficient. In most large efforts, I think it's rare when any project uses just one technology.

I could be wrong, but I find it unlikely that all the Perl code at your workplace will simply vanish. Also, just because you may be learning a new language doesn't mean that you have to drop Perl. It could be that some things you learn on the Java side will positively impact your Perl programming - stranger things have happened.

Aren't there other factors that are going to influence your decision? Are the benefits good? Do you enjoy the work environment, and like your coworkers?

It seems to me that you could look at this as a test to see how you deal with change. If it truly is the language that makes your job worthwhile, and that's the most important thing (and perhaps that is the key factor for some), then you might start looking around for other jobs. I'd recommend that you stick it out a bit, and give it a try first. If it turns out to be horrible then you can always look for other work. I think it's always worthwhile to learn an additional language (whether it's a programming language or otherwise). It gives you increased options and a broader view.