my $h = TheDb::PlainText->new();

##</code><code>##

package TheDB::PlainText;
@ISA = 'TheDb';
use TheDb;
use strict;
# do stuff

sub new {
  my $class = shift;
  my $self = $class->SUPER::new(@_);
  # whatever is specific to a text file here
  return $self;
}

# more stuff

1;

##</code><code>##

package TheDb;
# usual stuff

sub new {
  my $class = shift;
  my $self = {};
  # Whatever initializations
  return bless($self, $class);
}