G'day wiewa,

Welcome to the monastery.

While I don't see it having a direct bearing on the issue you raise, you'll find some related information in perl5180delta - New Restrictions in Multi-Character Case-Insensitive Matching in Regular Expression Bracketed Character Classes and perlrecharclass - Bracketed Character Classes.

Perhaps this is a bug somehow caused by that change.

In case, after reading those sections, you were wondering what 'LATIN SMALL LETTER SHARP S' looked like:

$ perl -Mstrict -Mwarnings -E '
    binmode STDOUT => "utf8";
    say "\N{LATIN SMALL LETTER SHARP S}";
'
ß
[download]

This appears to be the same character Corion talks about. [Update: Corion has confirmed (via /msg) that it is the character he's referring to. I said "appears" because, given the right font, uc('i') and lc('L') can "appear" to be the same character.]