Sound like you want to sort by parent and then by message ID. Something like this will work:

my @sorted = sort { $a->{parent} <=> $b->{parent} ||
                    $a->{messageId} <=> $b->{messageId} } @$testarray;

foreach (@sorted) {
  print "$_->{messageId}\t$_->{parent}\n";
}
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"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

#!/usr/bin/perl -wl
use strict;

## consider this a better way to go
## let %NODE be the representation of a typical record
## and in this case, our initial thread (aka a "root" node)

my %THREAD= ( node_id => 1,
              root_id => 0,  # 0 is the node_id of 'FooStuff', a "sect
+ion"
              parent_id => 0, # well the parent of a "root" node is a 
+section
              content => "how do I echo a value to the screen?",
              title => "How do I echo a value to the screen?",
            ,);

## at this point you've *selected* all nodes whose parent
## is %THREAD, ie, node_id => 1
## see, no need for recursive calls to the database
## it might be worth having a 'section' attribute
## if you want to be able to search through threads based on section

my @NODES = ( \%THREAD, # therad is #1
{ node_id => 2, root_id => 1, parent_id => 1,
    content => "i dunno",
    title => "re: How do I echo a value to the screen?" },

{ node_id => 3, root_id => 1, parent_id => 1,
    content => "well you use the function print",
    title => "re: How do I echo a value to the screen?" },

{ node_id => 6, root_id => 1, parent_id => 2,
    content => "don't say that",
    title => "re: re: How do I echo a value to the screen?" },

{ node_id => 9, root_id => 1, parent_id => 6,
    content => "you guys are dumb",
    title => "re: re: re: How do I echo a value to the screen?" },
           ,);

print_kids(\%THREAD, 1, @NODES);

exit;

# not the most efficient, but it will do
# (you're welcome to make with the improvements as always)

sub print_kids {
    my ($NODE, $t, @NODES) = @_;

    for my $ix(0..scalar @NODES) {

    next unless exists $NODES[$ix]->{parent_id};
    # due to sideffect of splice


        if( $$NODE{node_id} == $NODES[$ix]->{parent_id} ) {
            print "$t) ",
                  " " x (4 * $t ),
                  "( $NODES[$ix]->{node_id} ) ",
                  $NODES[$ix]->{title},
                  "\n>>>>>>>>",
                  ">" x (4 * $t ),
                  " $NODES[$ix]->{content}\n";

            my $N = splice(@NODES,$ix,1);

            print_kids($N, $t+1, @NODES);
        }
        elsif( $$NODE{node_id} == $NODES[$ix]->{node_id} ) {
            print "0) ",
                  "( $NODES[$ix]->{node_id} ) ",
                  $NODES[$ix]->{title},
                  "\n>>>>>>>>",
                  ">" x (4 * $t ),
                  " $NODES[$ix]->{content}\n";

            my $N = splice(@NODES,$ix,1);

            print_kids($N, 1, @NODES);
        }
    }

    return undef;
}

__END__
$>perl thread.txt.pl
0) ( 1 ) How do I echo a value to the screen?
>>>>>>>>>>>> how do I echo a value to the screen?

1)     ( 2 ) re: How do I echo a value to the screen?
>>>>>>>>>>>> i dunno

2)         ( 6 ) re: re: How do I echo a value to the screen?
>>>>>>>>>>>>>>>> don't say that

3)             ( 9 ) re: re: re: How do I echo a value to the screen?
>>>>>>>>>>>>>>>>>>>> you guys are dumb

1)     ( 3 ) re: How do I echo a value to the screen?
>>>>>>>>>>>> well you use the function print
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______crazyinsomniac_____________________________ Of all the things I've lost, I miss my mind the most. perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"

But the request specifically stated 'non-recursive' meaning this solution is not what was required.

rdfield

No recursive calls to a database. This one clearly doesn't recursively call to a database.

BMaximus

my $testarray = [
                 { messageId => 1,
                   parent    => 1 },
                 { messageId => 2,
                   parent    => 1 },
                 { messageId => 3,
                   parent    => 1 },
                 { messageId => 4,
                   parent    => 2 },
                 { messageId => 5,
                   parent    => 3 },
                 { messageId => 6,
                   parent    => 2 },
                 { messageId => 7,
                   parent    => 4 }
                 ];
                 


#wander down array, setting "children" field

foreach my $key (@$testarray) {
   
   next if ($key->{parent} eq $key->{messageId}); # skip root nodes
   push @{$$testarray[$key->{parent} - 1]->{children}}, $key->{message
+Id};
   # messageIds are 1-relative, arrays are 0-relative
}   


# build up a sorted array of elements...

my @newstack;

# a. retrieve each element in turn
foreach my $message (@$testarray) {
# b. if parent = messageid, push onto new stack, and go back to the be
+ginning
  if ($message->{parent} eq $message->{messageId}) {
     push @newstack,$message;
     next;
  }
  # c. insert it into the new stack, just after the parent
  for (my $loop = scalar(@newstack) - 1;$loop >= 0;$loop--) {
     #d. if the last element IS the parent, just push it and move on
     if ($newstack[$loop]->{messageId} eq $message->{parent}) {
        push @newstack,$message;
        $loop = -1;
        next;
     }
     #e. move the stack down a place
     $newstack[$loop + 1] = $newstack[$loop];
     #f. insert new value, if appropriate and trip out of loop
     if ($newstack[$loop - 1]->{messageId} == $message->{parent}) {
        $newstack[$loop] = $message;
        $loop = -1;
     }
  }
}
#print the results
print "Message\tParent\n";
print  $_->{messageId} . "\t" . $_->{parent} . "\n" for @newstack;
[download]

produces the results:

Message Parent
1       1
3       1
5       3
2       1
6       2
4       2
7       4
[download]

rdfield

update of course you could always retrieve the messages from the database in the correct order in the first place:

select messageId,parent
from messages
start with messageId = 1
connect by prior messageId = parent
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