diag
             diag(@diagnostic_message);

           Prints a diagnostic message which is guaranteed not to interfere with test output.  Like
           "print" @diagnostic_message is simply concatenated together.

           Handy for this sort of thing:

               ok( grep(/foo/, @users), "There's a foo user" ) or
                   diag("Since there's no foo, check that /etc/bar is set up right");

           which would produce:

               not ok 42 - There's a foo user
               #   Failed test 'There's a foo user'
               #   in foo.t at line 52.
               # Since there's no foo, check that /etc/bar is set up right.