A good start is to register your device online on purchase. Pioneer warranty is global, and the valid coverage depends on the region origin and whether its new or refurbished.
The proof of purchase is needed for making sure its a legit unit, and to know the official start date of the warranty. It does not need to be purchased from an authorized reseller but the warranty work will be charged internally to the appropriate region service center.
The original region warranty cover is applicable, as they differ from region to region, for example, for US new products its 1 year but some European countries its 2 years. If its a refurbished unit, its 90 days.
As mentioned earlier, unfortunately some locations might think you are not entitled for coverage, which is probably based on employee assumptions, lack of interest to deliver good service, or the mere fact that it might be the non-standard procedure for them. They will need to file an internal request and wait for the cost to be paid by another region. Hence the tendency to reject service and avoid the hassle. It does not however reflect your rights.