You are setting the deadline option with -w to 1 second. Which will terminate the ping after one second regardless of how many packets have been transmitted and received. If you increase this default you should get the five results you expect.

"Please do not suggest to use Net::Ping ,I am using system ping command for some reasons."

You will getter better responses from the monks if you can actually state your reasons. Using Net::Ping is generally going to be the better option compared to shelling out to system ping. If you want to shell out or reinvent the wheel you need a good reason to do so. Without a reason this comes across as homework.