The behaviour of lc and such can vary based on how the string is stored internally. This is a bug, but it can't be fixed due to historical reasons. You can work around the problem by switching the internal storage format of the string.

use open ':std', ':encoding(UTF-8)';  # UTF-8 terminal

my $s = "\xDC";
utf8::ugprade( $s );  # Use Unicode semantics
print lc($s), "\n";
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Perl 5.12 has a pragma to control the behaviour of lc.

use open ':std', ':encoding(UTF-8)';  # UTF-8 terminal
use feature 'unicode_strings';  # Or "use 5.012;"

my $s = "\xDC";
print lc($s), "\n";
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