use Tree (); 
my %nodes; 
while (<>) { 
my ($child, $parent) = split /:/; 

my $parent_node = $nodes{$parent} ||= Tree->new($parent); 
my $child_node = $nodes{$child} ||= Tree->new($child); 
$parent_node->add_child($child_node); 

} 

my @roots = grep { $_->is_root } values %nodes; 

die("Invalid data: Multiple roots\n") if @roots > 1; 
my $root = $roots[0];