Finally, I would be glad to verify your second claim if you can provide the input string upon which rollahead and unroll return different results. I believe such a string does not exist.

Actually, it was your own benchmark:

#! perl -slw
use strict;
use Benchmark ':all', ':hireswallclock';

my $x = "#@##@###@####@#####@";
my $y = reverse $x;
my $z = "$x$x$x$y$y$x$x$y$y$y$y$y$x$x$x$x";

our %results;
cmpthese -3, {
    unroll    => sub {
       my @x = ([unroll($x)],[unroll($y)],[unroll($z)]);
       $results{ unroll } = \@x;
    },
    rollahead => sub {
       my @x = ([rollahead($x)],[rollahead($y)],[rollahead($z)]);
       $results{ rollahead } = \@x;
    },
    lookbehind => sub {
       my @x = ([lookbehind($x)],[lookbehind($y)],[lookbehind($z)]);
       $results{ lookbehind } = \@x;
    },
};

printf "%20s => %s\n", $_,  "@{[ map{ qq[@$_] } @{ $results{ $_ } } ]}
+" for keys %results;

sub unroll {
    my @x = $_[0] =~ /(?:^|@)((?:##|#@|[^#@])*)/g;
    for(@x){
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

sub rollahead {
    my $x = shift;
    $x = reverse $x;
    my @x = reverse(split/\@(?=(?:##)*(?!#))/,$x,-1);
    for(@x){
        $_ = reverse;
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

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

Output (read'em and weep:):

C:\test>junk8
             Rate     unroll  rollahead lookbehind
unroll     3096/s         --        -3%       -33%
rollahead  3175/s         3%         --       -32%
lookbehind 4639/s        50%        46%         --
           rollahead => @# #@## ##@  ##@## #@# # @# #@## ##@@# #@## ##
+@@# #@## ##@ ##@## #@# @##@## #@# # # #@## ##@@# #@## ##@ ##@## #@# @
+##@## #@# @##@## #@# @##@## #@# @##@## #@# # # #@## ##@@# #@## ##@@# 
+#@## ##@@# #@## ##@

              unroll => @# #@## ##@  ##@## #@#  @# #@## ##@@# #@## ##@
+@# #@## ##@ ##@## #@# @##@## #@# # # #@## ##@@# #@## ##@ ##@## #@# @#
+#@## #@# @##@## #@# @##@## #@# @##@## #@# # # #@## ##@@# #@## ##@@# #
+@## ##@@# #@## ##@

          lookbehind => #@## ###@#### #####@  #####@#### ###@## # #@##
+ ###@#### #####@#@## ###@#### #####@#@## ###@#### #####@ #####@#### #
+##@## #@#####@#### ###@## ## ## ###@#### #####@#@## ###@#### #####@ #
+####@#### ###@## #@#####@#### ###@## #@#####@#### ###@## #@#####@####
+ ###@## #@#####@#### ###@## ## ## ###@#### #####@#@## ###@#### #####@
+#@## ###@#### #####@#@## ###@#### #####@
[download]