Try Test::MockObject::Extends:

use Test::MockObject;
use Test::MockObject::Extends;
use Test::More 'no_plan';

use strict;

Test::MockObject->fake_module( 'Foo' ); 
my $x = Test::MockObject::Extends->new( 'Foo' );

$x->mock( 'bar', sub { 2 } );

is( $x->isa('Foo'), 1 );
is( $x->bar('Foo'), 2 );
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