I believe your syntax for initializing your class is incorrect in sub new
Try the following in its place

sub new {
    my $class = shift;
    my $self = { };
    bless($self, $class);
    $self->init(@_);
    return $self;
}

sub init {
    my $self = shift;
    my ($board, $index, $level, $links ) = @_; 
    $self->{board} = $board;
    $self->{index} = defined( $index ) ? $index : 0;
    $self->{level} = defined( $level ) ? $level : 0;
    $self->{links} = defined( $links ) ? $links : [];
}
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UPDATE
my apologies for this post. I was not aware that the syntax used to init an object as used here was valid. I have always initted my objects in the way I provided. Many sorrows for the confusion.

davidj