I think you might be able to do it using autobox and overload. Something like this I think:

package REF;
use overload q("") => sub { die "Attempt to stringify reference" };

package main;
use autobox 'REF' => 'REF', DEFAULT => undef;

...
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You need at least perl 5.8.1 for this though. That's why I can't test the code myself. (I should update one of these days ...)