the esszett is a strange character (well, it is German) as it uppercases to 'SS'. This happens with many characters of other languages too. The standard Perl uc just leaves it there when uppercasing.

I suspect you use uppercase for case-insensitive comparison - which is not correct. foldcase would be the way to go as fc "ß" is indeed "ss".

Otherwise, maybe uc fc produces your desired result?