function newMonitoredVariable(name, value)
    Runtime:dispatchEvent{name="Variable"; variable=name, value=value}
    return function(...)
        if select('#', ...) == 0 then
            return value
        end
        local old = value
        value = ...
        Runtime:dispatchEvent{name="Variable"; variable=name, value=value, oldValue=old}
        return old
    end
end

Runtime:addEventListener("Variable", function (event) _G[event.variable] = event.value end)

set_fnord = newMonitoredVariable("fnord", 5)

print(fnord) -- prints 5

set_fnord("kumquat")

print(fnord) -- prints kumquat