Intuatively, you want to use split '(?<=(?:##)+)\@', $s;; but that gets you:

[Variable length lookbehind not implemented in regex; ...
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So how to achieve a variable length lookbehind? Here's one way:

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

print for split '(?:(?<=[^#]####)|(?<=[^#]##)|(?<!#))[@]', $s;;
#@##
###@####
#####@
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Of course the downside is that you need to include a case for each length of lookbehind which quickly gets unweildy:

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