my @aoa1 = ( [ 'a', 'b', 'c' ],
             [ 'd', 'e',  '' ],
             [ 'f', 'g',  '' ],
             [ 'h',  '',  '' ] );

sub combinations {
    return if ! @_;

    my @array = grep { $_ ne '' } @{shift()};

    my @subs = combinations( @_ );
    if ( ! @subs ) {
        return map { [ $_ ] } @array;
    }

    my @out;
    foreach my $item ( @array ) {
        foreach my $sub ( @subs ) {
            push @out, [ $item, @$sub ];
        }
    }

    return @out;
}

my @aoa2 = combinations( @aoa1 );