use Math::Combinatorics

my @data = ('a', 'b', 'c', 'd', 'e', 'f', 'g');
my $nmer = 3;
my $sampling_density = 2;

my $combinations = Math::Combinatorics->new(
        count   =>      $nmer,
        data    =>      [ @data ],
        );
my $to_skip = 0;

while (my @subset = $combinations->next_combination()) {
        if (0 == $to_skip) {
              $to_skip = int(rand($sampling_density));
              Process(@subset);
              }
        else {
              --$to_skip;
              next();
              }
        }