I ran into this one when I was doing some benchmarking. Specifically, I had something like this:

cmpthese(10000, {
   'orig' => sub { foreach my $x ( 0..9, 'a'..'v', 'A'..'V' ) { $x = M
+yMod::MapChar($x); }},
   'new'  => sub { foreach my $x ( 0..9, 'a'..'v', 'A'..'V' ) { $x = M
+yMod::MapChar2($x); }},
   }
);
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I first wrote it without the $x =, but was thinking that this might trigger some void optimization, so I added that part (without thinking about the aliasing implications obviously). This made those subs bomb out with "invalid parameter" errors when they received "10".