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:
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.]
In reply to Re: qr/STRING/ fails with certain lookbehind expressions