$ perl -CS -wE'no  feature qw( unicode_strings ); $_="\xEB"; utf8::downgrade($_); say /\w/||0; say uc;'
0
ë

$ perl -CS -wE'no  feature qw( unicode_strings ); $_="\xEB"; utf8::upgrade($_);   say /\w/||0; say uc;'
1
Ë

$ perl -CS -wE'use feature qw( unicode_strings ); $_="\xEB"; utf8::downgrade($_); say /\w/||0; say uc;'
0
Ë

$ perl -CS -wE'use feature qw( unicode_strings ); $_="\xEB"; utf8::upgrade($_);   say /\w/||0; say uc;'
1
Ë