$ perl x.pl
1..2
ok 1
ok 2
[download]

-       sub add      { reduce { $a + $b } @_ }
-       sub multiply { reduce { $a * $b } @_ }
+        use Scalar::Util qw/looks_like_number/;
+       sub add      { reduce { $a + $b } grep looks_like_number($_), 
+@_ }
+       sub multiply { reduce { $a * $b } grep looks_like_number($_), 
+@_ }
[download]

And, no, I don't particularly recommend this remedy. :-)