I need to write the java implementation for unpack function of perl

Do you really need to implement *all* of unpack's functionality??

I can think of several approaches (each of them having pros and cons):

1. Use Perl from Java
2. (re)use/fiddle with the C implementation of unpack via the JNI (Java Native Interface)
3. Implement only the unpack functionality that you need in Java. The Apache Commons could be a starting point.
4. Think again! Maybe Java is not the right choice?!

Anyway, not knowing the reason behind you wanting to implement a "jUnpack" it sounds a bit like a XY Problem.

Cheers,

Harry