use open ":std", ":locale";
use feature qw( say );

my $ss = "\xDF";
utf8::upgrade(   my $ss_u = $ss );
utf8::downgrade( my $ss_d = $ss );

{
   no feature qw( unicode_strings );
   say uc( $ss_d );                    # ß
   say uc( $ss_u );                    # SS
}

{
   use feature qw( unicode_strings );
   say uc( $ss_d );                    # SS
   say uc( $ss_u );                    # SS
}