package Bar;

sub override_Foo
{
  *Foo::new = \&Bar::new;
  *Foo::method2 = \&Bar::method2;
  # etc.
}

1;


##</code><code>##

package Bar;
use Exporter;
@Bar::ISA=qw(Exporter);
@Bar::EXPORT=qw(new method2);

sub override_Foo
{
  package Foo; # how dare we!
  Bar->import;
}

1;

# and in main:

use Foo;
use Bar;
Bar->override_Foo();