Ah, interesting. I missed that because it behaves differently depending on the use of features:

haj@vdesktop:~$ perl -M5.010 -C -e 'print uc chr 0xdf, "\n"'
ß
haj@vdesktop:~$ perl -M5.012 -C -e 'print uc chr 0xdf, "\n"'
SS
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