The most devious part is japhy's innocuous comment. $(...) syntax is hogwash! The $ starts the regex engine matching at the end of the string. Since $` is the portion preceding the current match, it gets set to the entire contents of $_. Now the (??{code}) zero-width assertion comes into play. $` >= $X is evaluated (as a string) and its result inserted into the regex. In Perl, boolean values stringify to "1" or "". So if $_ < $X, the comparison fails (returning ""), so the regex is simply /$/, which always succeeds. Otherwise, the comparison succeeds (returning "1"), and the regex is (equivalent to) qr'$1', which can never succeed.

Kudos (not Kudra) to you, blokhead! I'm very pleased to see you've deciphered my little regex. I'd make extra accounts to ++ you multiple times if that were within my morality. ;)
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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;