On a related note, which I consider required reading–

• Code that assumes roundtrip equality on casefolding, like lc(uc($s)) eq $s or uc(lc($s)) eq $s, is completely broken and wrong. Consider that the uc("σ") and uc("ς") are both "Σ", but lc("Σ") cannot possibly return both of those.
• Code that assumes every lowercase code point has a distinct uppercase one, or vice versa, is broken. For example, "ª" is a lowercase letter with no uppercase; whereas both "ᵃ" and "ᴬ" are letters, but they are not lowercase letters; however, they are both lowercase code points without corresponding uppercase versions. Got that? They are not \p{Lowercase_Letter}, despite being both \p{Letter} and \p{Lowercase}.
• Code that assumes changing the case doesn’t change the length of the string is broken.