require isa; # this should go away once this is in the core
package Foo;
isa Bar;

##</code><code>##

package isa;
use strict;
use warnings;

BEGIN {
    no warnings 'redefine';
    my $old = \&UNIVERSAL::isa;
    *UNIVERSAL::isa = sub {
        goto &$old if defined wantarray;
        no strict 'refs';
        push @{caller().'::ISA'}, @_;
    } #UNIVERSAL::isa
}
1;

##</code><code>##

isa Bar;

##</code><code>##

Bar->isa;