You are looking at a very old, but free version of the course. If you are interesed the most recent version, check it out on the Perl Maven site.
It is much more than a calculator, it is a language. Luckily we don't need to learn the whole language in order to to do simple calculations. Normally you execute 'bc' from the command line and then you type in your calculations. Pressing ENTER will do the calculation.