Add Perl starting code

We recently did your kata as a group and it was an excellent experience, thank you for sharing it.

This pull request adds Perl code as well. It is modeled after the Python code in that the first example is reasonable, second very verbose and third overly terse. Where appropriate I've changed Pythonisms into Perlisms.

The tests are more extensively refactored but still carry out the same tests as the Python code. Perl's testing approach is quite different to Python's. To run them:

``` shell
cd perl
prove test.pl
```

If `prove` is not available then `perl test.pl` or just `./test.pl` should work but with more verbose output. Either way all of these should be familiar to a Perl coder.