In the name of Golf, shave out the ?:

pop=~/^(?{$x=$y=0})((.)(??{"\Q$2"x$y})(?!\2)(?{$y=($x+=++$y)-$y})){3,}
+$/
[download]

_____________________________________________________
Jeff japhy Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;