my $ideal_avg_per_group = @list / 6;

# your solution
my $solution = [ 
 [1 1 1 2 3],
 [4 4 4 5 5],
 [6 6 6 7 7],
 [8 8 8]
];

my $this_score = 0;
for (@$solution) {
  my $num_items = @$_;
  $this_score += abs($ideal_avg_per_group - $num_items);
}