For some odd reason client wants to switch to Java.

Maybe you should try to talk to the customer, find out why she wants to switch, and maybe explain that since you know Perl better than Java, development in Java will take longer.

There are reasons why some people don't like java, but most of them are rather technical (for example that you need much more code to express something than in Perl, and some oddities like type erasure), but they are typpically not what a client is interested.

Typically clients want to have a product implemented in a way that's fast, cheap, reliable, finished early, easy to maintain, easy to find maintainers for (that's probably easier for Java than Perl, given the shear amount of Java programmers) etc.

You can use the knowledge for your own ends, and argue that you have better Perl expertise and than Java; however that might backfire if the client doesn't want to make a compromise, and rather choses a different consultant.