The answer really depends on what you're looking for in the drink.
1) The hotter the whiskey the more rapid the alcohol evaporates out and more of the bitter-notes shine through. Adding ice is cold + water which can also mask some of the whiskey characteristics while taking away some of the "harsh" fore taste of the whiskey.
*Whiskey stones could be a good alternative if you want cooler whiskey without the effects of the water.
2) The Perrier Water is going to add some fizz + bring the whiskey toward the temperature of the Perrier. The cooler the Perrier, the more the temperature drop on the finished product will be. Conversly, the hotter the Perrier is served, the more rapid the loss of CO2 will be.
*Freezing the Perrier (or being cold enough to almost freeze) can be dangerous because the H2O (32ºF/0ºC) freeze before the CO2 (-109.3ºF/-78.5ºC) causing a lot of pressure and potentially an pressurized explosion to occur.
3) The ideal ratio of Perrier+Whiskey is going to solely depend on the imbiber's taste. The more Perrier, the more fizz and more "watered down" the final drink. The less Perrier, the less fizz and more whiskey you will taste.
*Watered down is not being used in a negative connotation here. It is being used as a catch all to describe all of the ways water (carbonated or not) affect whiskey and other liquors.
The real answer is... Experiment!
- Johnny Walker, neat, at room temp.
- Johnny walker, neat, chilled
- Perrier at room temp.
- Perrier chilled
Then measure out ratios based on what your perceptions were.
If you liked the Johnny Walker closer to room temp, both should be closer to room temp. Adding measured volumes of each until you achieve your perfect ratio.
*An important note: The reason you don't add ice on top of the Perrier is because of the dilution when the ice melts. Therefore you could add whiskey-stones that chill without the dilution.