package Foo;
use strict;
our @ISA = 'Bar';
...

##</code><code>##

package Foo;
use strict;
use vars qw(ISA);
@ISA = 'Bar';
...

##</code><code>##

package Foo;
@ISA = 'Bar';
use strict;
...

##</code><code>##

package Foo;
use strict;
@Foo::ISA = 'Bar';
...

##</code><code>##

package Foo;
use strict;
use base 'Bar';
...