my $DB;

$DB = new TheDB('SQL');
$DB->fetch();

$DB = new TheDB('Text');
$DB->fetch();

$DB->{'DB'} = 'SQL';
$DB->fetch();

$DB = $DB->new('Text');
$DB->fetch();

$DB = new TheDB('Oracle');
$DB->fetch();

package TheDB;
use strict;

sub new {
    my ($proto, $DB) = @_;
    my $class = ref($proto) || $proto;
    my $self  = {'DB' => $DB};
    bless $self, $class;
  return $self;
}

sub fetch {
   if ($_[0]->{'DB'} eq 'SQL') {
       &_SQL_fetch(@_);
   }
   elsif ($_[0]->{'DB'} eq 'Text') {
       &_Text_fetch(@_);
   }
   else {
       die "Unknown class ".$_[0]->{'DB'};
   }
}

sub _SQL_fetch {
    print "Using SQL method\n";
}

sub _Text_fetch {
    print "Using Text method\n";
}