group                   =  groupBy (==)

groupBy _  []=  []
groupBy eq (x:xs)=  (x:ys) : groupBy eq zs
                           where (ys,zs) = span (eq x) xs

span p [] = ([],[])
span p xs@(x:xs')
    | p x = (x:ys, zs)
    | otherwise = ([],xs)
        where (ys,zs) = span p xs'

##</code><code>##

sub group {
    map{
        pack 'C*', @$_
    } @{ +reduce {
        defined $a->[0] && $a->[-1][0] && $a->[-1][0] == $b
            ? push @{ $a->[-1] }, $b
            : push @{ $a }, [ $b ];
        $a;
    } [], unpack 'C*', shift }
}

##</code><code>##

print for group 'ZBBBCCZZ';
Z
BBB
CC
ZZ