use v6;
my @CELLS= 'A-1', 'A-2', 'A-3', 'A-4', 'B-5', 'B-6', 'C-7', 'C-8';

my %hash = @CELLS.map({ my ($k, $v) = .split('-'); $k => $v})\ 
                 .categorize(*.key)\                 # group by key
                 .map({; .key => [.value>>.value]})  # list of pairs to list of values
                 ;

say %hash.perl;