package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A' or $_[0]->SUPER::isa(@_);
}

##</code><code>##

package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(A B);

##</code><code>##

package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A';
}