a `pe4rl6 -c` error to fix

[Todd Chester]

Dear Perl Developers,

Would you please fix this `perl6 -c` checker error?
$ perl6 -v
This is Rakudo version 2018.05 built on MoarVM version 2018.05
implementing Perl 6.c.


The checkers passes this line with Syntax OK

     $ReturnStr, $CurlStatus = CurlDownloadFile $FileAddr, 
$BaseFileName, %MaxTime<'MaxTime4'>;


But actually running the line throws

     Useless use of $ReturnStr in sink context


The checker should throw an error if the line is "useless"

Many thanks,
-T