use Graph;
use Graph::TransitiveClosure::Matrix;

...

sub print_union {
    my ( $node, $signal ) = @_;

    my $g = Graph->new();
    for my $i ( 1 .. $#{$node} - 1 ) {
        for my $j ( 2 .. $#{$node} ) {
            if ( defined( $signal->[$i][$j] ) and ( $signal->[$i][$j] > 0 ) ) {
                $g->add_edge( $i, $j );
            }
        }
    }

    my $tcm = Graph::TransitiveClosure::Matrix->new( $g, reflexive => 0 );

    print "    ";
    for my $i ( 2 .. $#{$node} ) {
        printf "%2i ", $i;
    }
    print "\n    ";
    for my $i ( 2 .. $#{$node} ) {
        printf "-- ",;
    }
    print "\n";
    for my $i ( 1 .. $#{$node} - 1 ) {
        printf "%2i] ", $i;
        for my $j ( 2 .. $#{$node} ) {
            printf "%2s ", ( $tcm->is_transitive( $i, $j ) ) ? "X" : " ";
        }
        print "\n";
    }
}