Even if you are the maintainer of the Tcl version, its worth rereading the code to look for problems. Porting a package is a good opportunity to refactor the code. To refactor code is to apply a set of changes to the internal structure of the software to make it easier to understand and cheaper to modify without changing its observable behavior. When moving code from Tcl to Java, you need to understand what that behavior is and find the most efficient and maintainable means of expressing it in Java. Ideally, you should start by writing unit tests and functional tests for any code that you are writing.
Unit tests are tests of individual units of code, e.g. object methods.
They should encode the known correct results based on the parameters to the method. This allows the test to be run automatically as part of a daily build test. Functional tests test the overall functionality of a piece of software. An example of a functional test is the TemplatingTest which ensures that the Templating functionality is running correctly. Now that you're familiar with the package and you've got CVS setup, you' re ready to start the porting.