It can't be that you get "ABCd" when it's "ABCD" that is there. Try again, and this time, let Perl help you debug it. Add code like:

print "Oops, '$fileName' ne '\U$fileName\E' at line $." if $fileName n
+e uc($fileName);
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Surely, it'll have to warn you at some point, otherwise using the uc() wouldn't make a difference.