Untested and (in part) schematic code follows:

sub new
{
    my $class = shift;
    my $self = { children => [] };
    bless $self, $class;
}

sub addSubBlock # (newblock)
{
    my $self = shift;
    my $block = shift;
    push @{$self->{children}}, $block;
}

sub traverse_preorder
{
    my $self = shift;
    my @children = ();
    foreach my $child (@{$self->{children})
    {
        push @children, $child, $child->traverse_preorder;
    }
    return @children;
}
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OK, so I'm using a hash reference for the object data. I'm only using that for ease of illustration.

Updated:renamed traverse_inorder to traverse_preorder. D'oh!