$D = new DATASTORE 'dir', 'filename';
while($_ = $D->getline) {
  [...]
}

##</code><code>##

package DATASTORE;
use parent 'IO::File';

sub new {
	my ($class, @args) = @_;
	my $filename;

	[...]

	$self = $class->SUPER::new($filename, 'r');
	return $self if defined $self;
}