Why is this not sane?
Of course you can do whatever you want, but why might this not be a good idea? Perl is quite large. The entire Perl interpreter is required to run any piece of Perl code. Thus, if your app requires Perl, it will take up a huge amount of storage space. Furthermore, it will probably be quite processor-intensive to run. Java is already expensive to run, and you will be adding the Perl executable on top of that.
Of course, many (most? (all?)) tasks are much easier to accomplish in Perl than Java. That being said, Java can do pretty much anything. It may be more annoying to write it in Java, but it is almost certainly still preferable to stick with one widely available environment for your project. The cost of adding Perl is just too high on a mobile device with limited resources.
When's the last time you used duct tape on a duct? --Larry Wall