package class_X;
require DBI;
@ISA = qw( DBI );
sub new {
     my( $class, $dns, $username, $password ) = @_;
     my %attr = ( PrintError => 1, RaiseError => 0, AutoCommit => 1 );
     my $self = DBI->connect( $dns, $username, $password, \%attr ) or return;
     push @ISA, ref $self if( not grep { $_ eq ref $self } @ISA );
     return bless $self, $class;
}

sub DESTROY {
     my( $self ) = @_;
     $self->disconnect;
}
1;

package class_Y;
require class_X;
@ISA = qw( class_X );

sub new {
     my( $class ) = @_;
     my $dns = "DBI:mysql:database=XXX";
     my $user = "XXX";
     my $pwd = "XXX";
     my $self = class_X->new( $dns, $user, $pwd );
     return bless $self, $class;
}

sub DESTROY {
     my( $self ) = @_;
     $self->disconnect;
}
1;