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sub new {
    my($proto, $fields) = @_;
    my($class) = ref $proto || $proto;
    my $self = bless [], $class;
    ...

sub new
  {
...
  my $self = { @atom_fields, @_ };
  $self = bless ($self, ref($proto) || $proto);
  ...

sub new {
    my($proto, $fields) = @_;
...
    # make a copy of $fields.
    bless {%$fields}, $class;
}