my @array_2D = blablabla
my ($tree, $tmp);

foreach my $row (@array){
    $tmp = extend_MTree($row);
    $tree = add($tmp, $tree);
}

$tree now contains the hash!

sub extend_MTree($)
  {
    my ($a) = @_;

    my %H_tmp;

    if( $#{$a} == -1)
      {
	return 1;
      }

    my $elmt = shift @$a;
    $H_tmp{$elmt} = extend_MTree($a);
    return \%H_tmp;
  }

sub add()
  {

    my ($tmp, $res) = @_;

    foreach my $key (keys %$tmp)
      {

	# this is a new path, add here and leave
	if(!defined($res->{$key}))
	  {
	    $res->{$key} = $tmp->{$key};
	    return $res;
	  }

	# this part of the path exists, recurse
	$res->{$key} = add($tmp->{$key}, $res->{$key});
	return $res;
      }