You should not see this as a betrayal. A programming language is not a person, so it cannot be betrayed. As long as your project will not be harmed by doing the work in Java, I don't see anything wrong with recommending it.

There are a lot of people out there who practically become married to technologies that it becomes there own downfall. Look at the mainframe or VB programmers out there. Where I live, employers typically receive 200 to 300 resumes for positions for the mainframe and VB. Considering how competetative the job market is right now, I think its a good thing to broaden your skills. Who knows, maybe you'll find something in Java that helps you to improve your Perl skills.