my @input = ( 'a' .. 'e' );
my $max_subset = 3;
my $iter = NestedLoops(
    [ sub { [0] }, ( sub { [ 0 .. 1+$_ ] } ) x $#input ],
    sub {
        return -1  if  $max_subset < $_;
        return  1  if  @_ == @input;
    },
);
my @slots;
while(  @slots = $iter->()  ) {
    my @subsets = ('') x @input;
    $subsets[$slots[$_]] .= $input[$_]
        for 0 .. $#input;
    print "@subsets\n";
}