why not generalize your problem slightly and use Jarko Heitaniemi's Graph packages to do the work of the postorder traversal for you?

i am normally reluctant to suggest any reengineering of code presented, but i have personally saved so much time using Graph::Base, Graph::Directed and Graph::DFS that i feel compelled to sketch out the following:

use Graph::Directed;
use Graph::Base;
use Graph::DFS;

my $g = Graph::Directed->new();

growtree($g, $mydatastruct); # create the tree rooted in $g

my $search = Graph::DFS->new($g); # for the search...

while (my $vertex = $search->next_postorder()){
    my $info = $g->get_attribute('myattrib',$vertex);
    # do stuff with $info...
}

sub growtree{
    my ($g, $mydatastruct) = @_;
    my $data = # ...get stuff from $mydatastruct;
    $g->set_attribute('myattrib',$g, $data);
    my @newedges = # ...get new edges from $mydatastruct
    $g->add_edge($g,$_) for @newedges;
    my @newvertices = grep {! $g->has_vertex($_)} @newedges;
    map { growtree($_,$mydatastruct) } @newvertices;
}
[download]

won't save you time immediately, but very handy in the medium & long term.

hope it helps;