i don't even know what a perfect square is, but my idea was
2^k such that k is even = 4, 16, 64...
3^k such that k is even = 9, ...
5^k such that k is even = 25, ...
7^k such that k is even = 49...,
OH WAIT, now I know what a perfect square is, yeah I get it, lol that's pretty cool...