I thought about adding a test for this ticket, but now wonder what the expected output for the evaluation is.
As Moritz showed a current Rakudo complains about 'Useless use of "+" in expression "$a + $b" ...'. In masak's original example the warning was about "Useless use of variable $a ...". I tried with a Rakudo from June 2015 and that one complained about 'Useless use of "," in expression "$a, $b = $b, $a +" ...'. So many different useless uses ... On the other hand there are quite a few tests in S32-exceptions/misc.t to make sure that we get that kind of warning from 'perl6 -e' at all -- cmp. commit 05f4cd93 in roast. Maybe we could close this ticket without adding a test?
