|Don't ask to ask, just ask|
I kind of see the problem you perceive, though I'd much prefer to handle it with defined:
Double reference to $employee->salary or not; I bet it is a) more efficient than your module; b) far less prone to misunderstanding or accidental disablement than your scary-action-at-a distance module.
Personally; I think that instead of initialising things as unknown, you'd achieve a better effect by initialising them as #NaN>.
Indeed, I can see the case for a pragma to cause undef to be treated as #NaN in numeric contexts.
A lexically scoped use undefAsNaN; could be very useful I think.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.