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.
diag prints out a message along with the rest of the output.
Use it for whatever extra output in order to ensure that your printouts will not interfere with future changes in the test environment modules (such as Test::Harness).
diag "We are going to test the Foo-Bar device now";
# We are going to test the Foo-Bar device now