It is very important to write good unit tests for software that is non-trivial and that is relied on by other pieces of software.
Often the logic of the software can easily be covered by the native testing facilities of the programming language, like JUnit for Java or, much less well known but available, CUnit for C. When a lot of framework code is involved or third party libraries are used heavily, there is almost no other way for certain tests, because setting up the environment cannot easily be achieved elsewhere.
But we also encounter cases where writing good unit tests in the same language as the library itself becomes a pain. We procrastinate the issue of writing them and end up with way too little or no unit tests at all. A lot of software deals with doing some calculations or transformations of numbers and strings, usually a lot of numbers and a lot of strings. Now Strings are the strength of the Perl programming language and are not really implemented very well or at least not very powerful or easy to use in most other languages. Specifically the String facilities of Perl are much superior to those of Java and C. Off course our software needs to perform well, it needs to integrate into the environment and follow the global corporate software standards, so Java or C or some other programming language is the choice that should not be challenged here for the productive software. But some tests of the functionality can more easily be achieved by iterating over some input data and creating output of the input data combined with the results. This can be perl code already or something really easy to parse. Perl is really the tool that can parse almost anything, but we do not really want to be distracted by unnecessary work but get our job done. So something like generating Perl code or CSV or YAML or JSON, but please not XML if not really needed, should do. Then we can pipe the output to perl or to a perl script and this will tell us, if everything is ok. When we know our platform, it can even be done that the Java- or C-Unit-test stores the output to a file or pipe and calls the Perl script on it and fails or succeeds depending on its output.
When it comes to numeric types, Ruby is very strong. It has unlimited size integers by default, which can be casted to n-bit-integers using constructions like
it has Rational, LongDecimal (as an external gem) and Complex and is easily extendible.
Usually we can expect that corporate constraints on which tools and programming languages may be used are less restrictive when it comes to unit tests. Integrating this on a continuous integration platform is a job that needs to be addressed but it is worth the effort, if a lot of tests become easier with this approach. And doing tests in another language makes tests more credible.