...
sub lookbehind {
    my $x = shift;
    split '(?:(?<=[^#]####)|(?<=[^#]##)|(?<!#))[@]', $x;
}

C:\test>junk8
Subroutine Benchmark::mytime redefined at c:/Perl/lib/Benchmark.pm line 459.
Benchmark:
timing 10000 iterations of
 lookbehind, rollahead, unroll
...
lookbehind: 2.34183 wallclock secs ( 2.19 usr +  0.00 sys =  2.19 CPU) @ 4570.38/s (n=10000)
 rollahead: 3.46343 wallclock secs ( 3.23 usr +  0.00 sys =  3.23 CPU) @ 3092.15/s (n=10000)
    unroll: 3.51775 wallclock secs ( 3.33 usr +  0.02 sys =  3.34 CPU) @ 2990.43/s (n=10000)

             Rate     unroll  rollahead lookbehind
unroll     2990/s         --        -3%       -35%
rollahead  3092/s         3%         --       -32%
lookbehind 4570/s        53%        48%         --

##</code><code>##

          rollahead => @# #@## ##@  ##@## #@# # @# #@## ##@@# #@## ##@@# #@ ...
             unroll => @# #@## ##@  ##@## #@#  @# #@## ##@@# #@## ##@@# #@# ...

##</code><code>##

          lookbehind => #@## ###@#### #####@  #####@#### ###@## # #@## ###@#### ...