sub visit_preorder {
   my ($cb, node, $depth) = @_;
   $depth ||= 0;
   $cb->($node, $depth);
   visit_preorder($cb, $_, $depth+1) for $node->children();
}

sub visit_postorder {
   my ($cb, $node, $depth) = @_;
   $depth ||= 0;
   visit_postorder($cb, $_, $depth+1) for $node->children();
   $cb->($node, $depth);
}

visit_preorder(sub {
   my ($node, $depth) = @_;
   ...
}, $root);
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It's odd that the module doesn't provide this for you.

What's wrong with the solutions we've already customised for you?

Thanks. The module does provide an example for output, but I couldn't figure out how to implement it in the example shown here. For example, here is what the module gives:

 my @nodes = $tree->traverse( $tree->PRE_ORDER );
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The example I was working off of:

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];
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Basically, I didn't know what to do since I couldn't find a tree object? I'm sure I am missing all sorts of obvious signs in the code -- I am just too much of a newby to fully understand it. Sorry.

The module does provide an example for output,

The module's traverse is useless in most circumstances since it provides a flat list.

Basically, I didn't know what to do since I couldn't find a tree object?

Every node is a Tree object.

Hi, Can you explain what the pieces of this does

sub visit_preorder {
   my ($cb, node, $depth) = @_;
   $depth ||= 0;
   $cb->($node, $depth);
   visit_preorder($cb, $_, $depth+1) for $node->children();
}
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For example, what is '$cb' ? What I am trying to do is output the tree in a flattened format once the tree is built. Something like : Fruit|Apple|Granny Smith.. Fruit being the parent of Apple, Apple being the parent of Granny Smith..

For example, what is '$cb'

The first argument. It's expected to be a code reference.

What I am trying to do is output the tree in a flattened format once the tree is built.

Based on the code from Re: Tree path analyzer regex problem (maybe other issues)?, a reply in an older thread of yours,

sub leaves {
    my $node = shift;
    my @children = $node->children();

    if (!@children) {
        print(join('|', @_), "\n");
    } else {
        for my $child (@children) {
            leaves($child, @_, $child->value());
        }
    }
}
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