Well, that is not exactly how i would have solved the problem, but if it works for you, then good. If you are curious as to how i would solve the problem, then read on.

Use A Database!

Of course, seeing as how you have come along this far with your solution, switching now probably seems impossible. I understand that completely, but here is some code that keeps track of "threads" via DBI.pm and CGI.pm. I used the MySQL database with the following table creation syntax:

CREATE TABLE node (
  id int(10) NOT NULL auto_increment,
  name varchar(64) NOT NULL default '',
  parent int(10) default '0',
  PRIMARY KEY  (id)
);
[download]

use strict;
use warnings;

use DBI;
use Data::Dumper;
use CGI::Pretty qw(:standard *ul);

my %node;
my $dbh = DBI->connect(
    qw(DBI:vendor:database:host user pass),
    {RaiseError=>1}
);

print header(),start_html('messages'),
    h1('messages'),start_form,
    'New thread title: ', textfield('name'),br,
    'Parent thread: ', textfield('parent'),br,
    submit('go'),end_form,
;

insert_message(param('name'),param('parent')) if param('go');

display_messages();
print end_html;

$dbh->disconnect;

sub display_messages {
    my $sth = $dbh->prepare('select id,name,parent from node');
    $sth->execute;
    while (my $row = $sth->fetchrow_hashref()) {
        $node{$row->{id}} = { name => $row->{name} };
        push @{$node{$row->{parent}}->{children}}, $row->{id}
            if $row->{parent};
    }
    # uncomment this to see the datastructure
    #print pre(Dumper \%node),hr;

    print start_ul;
    display_thread($_) for sort {$a<=>$b} keys %node;
    print end_ul;
}

sub display_thread {
    my $id = shift;
    my $node = delete $node{$id};
    return unless $node;

    print li("$id: " . $node->{name});
    return unless $node->{children};

    print start_ul;
    display_thread($_) for @{$node->{children}};
    print end_ul;
}

sub insert_message {
    my ($name,$parent) = @_;
    my $sth  = $dbh->prepare('
        insert into node(name,parent) values(?,?)
    ');  
    $sth->execute($name,$parent);
}

# this sub is not used, but i'll leave it anyway ;)
sub get_last_id {
    return $dbh->selectcol_arrayref('SELECT last_insert_id()')->[0];
}
[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)