my @heads;
while(my ($key, $value) = each(%hash)) {
    # does this msg have a parent?
    if (my $parent = %hash{$value->{inreply}}) {
        # link the parent to this message
        push @{$parent->{nextmsg}}, $key;
    }
    else {
        # no parent so it must become a thread start
        push @heads, $key;
    }
}