sub make_combo_iter
{
    my $array_ref = shift;
    
    # last index of top level
    my $size = scalar( @{$array_ref} );
    
    # create array of last indices
    my @array_limits;
    for my $i (0..$size-1)
    {
        $array_limits[$i] = scalar( @{$array_ref->[$i]} ) - 1;
    }
    
    # one counter for each sub-array
    my @array_counter = (0) x $size;
    my $done = 0;
    my $iter = sub { 
        if ($done)
        {
            $done = 0;
            return undef;
        }
         
        my @next_combo;
        # compute @next_combo
        for my $i (0..$size-1)
        {
            push @next_combo, $array_ref->[$i][$array_counter[$i]];
        }
         
        # increment counters, set $done if the last one rolls over
        my $rollover = 0;
        for my $c (0..$size-1)
        {
            if (++$array_counter[$c] > $array_limits[$c])
            {
                $array_counter[$c] = 0; # reset
                $rollover = 1;          # carry
            }
            else
            {
                $rollover = 0;  # no carry
                last;           # no more increments
            }
        }
        
        # if the last array rolled over, set the done flag
        if ( $rollover )
        {
            $done = 1;
        }
         
        return \@next_combo;
    }; # anonymous iterator sub
    return $iter;
} # make_combo_iter