Of course if you want to do something contingent on value change...then it seems perl only offers a tied-type solution.

The documentation Lvalue subroutines does make a reference to Sentinel, which, from a quick look at the docs, appears to implement something like a tied scalar, in XS.

The tied scalar solution appears to already have been implemented with Sub::Lvalue, and in fact a search for "lvalue" on CPAN will show that the wheel has been reinvented several times, including making object accessors lvalues. One of the most comprehensive ones appears to be Contextual::Return (which implements its LVALUE/RVALUE blocks using tied scalars, too).