Of course, right after I said I was "done" with this topic, I realized that the "recursive lookbehind" technique can be used on its own, without all that complicated lookahead logic, since that's what the regex engine and backtracking are for ;-)

qr{
    (?<cur> . )
    #(?(?= (?&lookback) | .* \g{cur} )
    #    (?{print "cur($+{cur}) seen before or after\n"})
    #    (*FAIL)
    #)
    # simpler:
    (?! (?&lookback) | .* \g{cur} )
    (?{ print "post: $-[0]<$&>$+[0]\n" })
    (?(DEFINE) (?<lookback>
        (?<=
            (?<prev>  (?! \g{cur} ) . )
            (?{ print "  prev: <$+{prev}> / $-[0]<".($&//'-').">$+[0]\
+n" })
            (?=(?&lookback)) .
        |
            (?= \g{cur} ) . .
        )
    ) )
}msx
[download]

Update: Another smaller simplification ((*FAIL) isn't needed).