sub new {
  my $class = shift;
  $class = ref($class) if ref($class);
  my ($sequence,$type) = @_;
  my $self = bless {},$class;
  if (ref($sequence)) {
    croak "Can't initialize sequence from non-Sequence object.\n"
      unless $sequence->can('seq');
    %{$self} = %{$sequence};  # clone operation
  } else {
    croak "Doesn't look like sequence data" 
      unless $sequence=~/^[gactnu ]+$/i;
    $self->{'data'} = $self->_canonicalize($sequence);
    $self->{'type'} = $type || ($sequence=~/u/i ? 'RNA' : 'DNA');
  }
  return $self;
}

Example 5: Defining the new() constructor.