powerset

use feature qw(state);
use strict;
use warnings;

sub powerset($$&@) {
    my $bitmask = shift;
    my $limit = shift;
    my $callback = shift;
    my $bytes = @_/8;
    {
        my @indices = grep vec($$bitmask, $_, 1), 0..$#_;
        return if $callback->( $limit, @_[@indices] );
        ++vec($$bitmask, $_, 8) and last for 0 .. $bytes;
        redo if @indices != @_;
    }
}

my $bitmask = '';
my @seq = 1..21;
my $callback = sub {
    state $counter = 0;
    my $limit = shift;
    if (++$counter > $limit) {
        $counter = 0;
        print "limit reached\n";
        return 1;
    }
    print "[@_]\n";
    return 0;
};

my $limit = 6;
powerset \$bitmask, $limit, \&{$callback}, @seq;
powerset \$bitmask, $limit, \&{$callback}, @seq;