Ah, a twist. The common question is about permutations.

Nevertheless, I'd still use recursion. Say you need $n elements. Is the first element included? If yes, combine it with all combinations of $n-1 elements from the list of all of the following elements. If no, get all combinations of $n elements of the same sublist. Of course, you need to include every possible solution, which means walking every possible path.

Code!

sub combinations {
    my $n = shift;
    return [@_] if $n == @_;
    return () if $n > @_ or $n < 0;
    my $first = shift;
    my @r = ((map [ $first, @$_ ], combinations($n-1, @_)),
             combinations($n, @_));
    return @r;
}

use Data::Dumper;
print Dumper [ combinations(2, (0, 1, 2, 3)) ];
[download]

It appears to be working well.

Update: I'm pretty sure inserting

    return [] if $n == 0;
[download]

at the appropriate place, i.e. among the other return statements near the top, will improve efficiency quite a bit. It avoids doing a lot of useless recursion if all you want is the empty list as a singleton.

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