Typical solution for your problem is factory pattern. You don't have to play these dirty games with namespaces. Just create one more package:

package Bar;

use Foo::XXX;

sub create_foo {
    return Foo::XXX->new(@_);
}
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Idea is that you don't call constructors directly (i.e. Foo::XXX->new) but delegate it to third party (i.e package Bar). This way you don't have to put name of Foo:XXX package in your code. You can always create its instancies via Bar package:

use Bar;
my $foo = Bar->create_foo;
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If at some moment you decide to start using package Foo::YYY->new instead of Foo::XXX->new than the only place you have to modify is Bar::create_foo().

--
Ilya Martynov (http://martynov.org/)