When two goods are perfect substitutes, the indifference curve is a straight line. Examples of perfect substitutes might be a red pencil and a blue pencil or two nickles equals one dime. In the textbook case, a fall in the price of one substitute means that the consumer will only buy the cheaper good.
Are the Kindle Fire and iPad tablets perfect subs. If so, why don't I buy two and a half Kindles instead? I reckon that that I have a utility function for the two goods in the form of: U = 3iPad + 4Fire. I would always prefer the Fire but I reach diminishing marginal utility after the first consumption.
Along the same line, what would my demand function look like? I offer that my demand curve would be vertical at one fire. Other demand determinates like income, related goods, and expectations are irrelevant.