# in package ShoppingCart
@required_methods = qw(GetNext GetFirst Reset ...);

sub initialize {
  my $self = shift;
  my $IO = shift;
  croak "The first parameter to 'new ShoppingCart' must be an IO object!"
    unless ref $IO;
  foreach my $method (@required_methods) {
    croak("The IO object passed to 'new ShoppingCart' does not implement the necessary methods!")
      unless $IO->can($method);
  }
  $self->{IO} = $IO;
}
...