fix tests on 5.10 and 5.8

t/basic.t

@@ -5,7 +5,7 @@ no warnings::illegalproto;
 my $x = eval "sub (frew) { 1 }";
 is 1, $x->(), 'does not die on "bad" prototype';
 
-eval 'my $f = undef . "foo"';
+eval 'my $f = (undef) . "foo"';
 like $@, qr/uninitialized/, 'does not override uninitialized warnings';
 
 eval 'my $f = 5 + "foo"';