Well, one way to do it is to update the parent's hashref as you're looping through the data: 
# Get the data and store it in a message hash
while (<DATA>) {
  chomp;
  my ($id, $parent, $from, $date, $content) = split /,/;
  $msg->{$id} = {
    # id    => $id,  # only if you feel it's necessary
    thread  => $parent,
    from    => $from,
    date    => $date,
    content => $content,
    child   => [],
  };
  push @{ $msg->{$parent}[child] }, $id;
  # or even
  # push @{ $msg->{$parent}[child] }, $msg->{$id}
  # to save time/work/effort/etc
  # but then it might be necessary to store the id
  # in the hash itself
}

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

In reply to Re: Message threading by japhy
in thread Message threading by sschneid

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