Respond better how? If I've an object blessed into Ye::Olde::Proxy and it provides all the interface of Real::Deal, what can it do to inform blessed() that it should return Real::Deal instead of Ye::Olde::Proxy? Ye::Olde::Proxy would be too accurate since the proxy object wants to appear to everyone except itself to be a Real::Deal object. With ref(), it can install its own function into CORE::ref that knows how to lie properly. Is the idea that it's easier to get a lying blessed() function than it is to override CORE::ref()?

package Scalar::Util;

my $old_blessed = do {
    no strict 'refs';
    delete ${ "Scalar::Util::blessed" };
};
sub blessed {
    my $val = $old_blessed->(@_);
    if ( defined $val && $val eq 'Ye::Olde::Proxy' ) {
        return 'Real::Deal';
    }

    return $val;
}
[download]

Update: Fixed bug.

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My criteria for good software:

1. Does it work?
2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

Just an FYI, \&foo isn't a valid target for delete(). If your example even works, it's a bug.

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Being an mock object implies that it's blessed in the class that it's mocking.

It does? That's the way that the current mock object CPAN modules tend to do it - but it doesn't have to be. I've done mocking with subclasses, and with completely unrelated classes that duck type to the bit that I happen to be testing.