perldeveloper has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I've asked a similar question an hour ago in the ChatterBox, but I need more room to really explain it: Is there a way (a cpan module using XS, or anything) to tie a variable to an existing instance of an object, rather than specify the classname and its constructor parameters?

Basically, assuming a scalar, you'd have a tying-class (say TyingClass) (the class defining sub TIESCALAR) and a scalar variable (say $variable). With these two, you'd normally write something like:
tie $variable, 'TyingClass', @optionalConstructorParameters
Then, tied $variable would return the instance (created running TyingClass->TIESCALAR(@optionalConstructorParameters)).

But say I created my own instance of the TyingClass, named $tyingInstance (passing any set of constructor parameters). And then, I've changed the instance calling a stateful method on $tyingInstance (a method which depends on the state of its object), and this method actually changed the $tyingInstance object, in a way that cannot be reproduced by its constructor. How can one (if they can) tie my $variable to $tyingInstance, assuming TyingClass can be any Perl class, not specifically designed for this purpose.

Maybe there's a good reason why tying is done only in this way, so if there is one, I'd appreciate if someone took the time to explain it. Thanks.