I can understand there being discussion about the language at the start of a project, but you seem to be saying that the project has already started and the client wants to rewrite code in Java. Rewrite, do again. A case of "if it ain't broke then fix it anyway".

Or maybe the client just wants the rest of the project in Java, so now you have language compatibility problems. Switching languages for no good reason mid-project just does not make sense, these must be something more to it, surely?

There are non-technical reasons for choosing Java (or any other language), maybe the client is Sun or Oracle, in which case it would be understandable. I have seen some clients, commercial and state, that only employ programmers with some certification track qualification. Java has that and Perl does not. More likely though the client has a skewed view of Perl as a language. In some quarters Perl still has a reputation for not being a serious language, but more a hacking tool. Google "Perl success stories" might find what you are looking for.

Java is backed by a big corporate with pots of money, which makes the suites more comfortable. That's a completely false sense of security (as the current banking crisis shows) but its hard to fight.