perldoc -f fc says using lc to compare strings is "Wrong!" and that uc is "Also wrong!" and that we should use "fc" or a regex. Comparing lc, uc and fc shows that only lc actually works, while uc fails, and fc doesn't even exist. What's going on?

perl -le 'print $]'
5.028000

perl -CS -le 'print    "\x{1e9e}"'
ẞ

perl -CS -le 'print lc "\x{1e9e}"'
ß

perl -CS -le 'print    "\x{00df}"'
ß

perl -CS -le 'print uc "\x{00df}"'
ß

perl -CS -le 'print lc "\x{1e9e}" eq    "\x{00df}" ? 1 : 0;'
1

perl -CS -le 'print    "\x{1e9e}" eq uc "\x{00df}" ? 1 : 0;'
0

perl -CS -le 'print lc "\x{1e9e}" eq lc "\x{00df}" ? 1 : 0;'
1

perl -CS -le 'print uc "\x{1e9e}" eq uc "\x{00df}" ? 1 : 0;'
0



perl -CS -le 'print fc "\x{1e9e}" eq fc "\x{00df}" ? 1 : 0;'
String found where operator expected at -e line 1, near "fc "\x{00df}""
	(Do you need to predeclare fc?)
syntax error at -e line 1, near "fc "\x{00df}""
Execution of -e aborted due to compilation errors.



perl -CS -le 'print fc("\x{1e9e}") eq fc("\x{00df}") ? 1 : 0;'
Undefined subroutine &main::fc called at -e line 1.



perldoc -f fc
    fc EXPR
    fc      Returns the casefolded version of EXPR. This is the internal
            function implementing the "\F" escape in double-quoted strings.

            Casefolding is the process of mapping strings to a form where
            case differences are erased; comparing two strings in their
            casefolded form is effectively a way of asking if two strings
            are equal, regardless of case.

            Roughly, if you ever found yourself writing this

                lc($this) eq lc($that)    # Wrong!
                    # or
                uc($this) eq uc($that)    # Also wrong!
                    # or
                $this =~ /^\Q$that\E\z/i  # Right!

            Now you can write

                fc($this) eq fc($that)

            And get the correct results.