[Variable length lookbehind not implemented in regex; ...

##</code><code>##

print $s;;
#@##@###@####@#####@

print for split '(?:(?<=[^#]####)|(?<=[^#]##)|(?<!#))[@]', $s;;
#@##
###@####
#####@

##</code><code>##

print for split '(?:(?<=[^#]########)|(?<=[^#]######)|(?<=[^#]####)|(?<=[^#]##)|(?<!#))[@]', $s;;