Jelly, 11 bytes
µÆE;¬g/’µ#Ṫ
Background
Every positive integer k\$k\$ can be factorized uniquely as the product of powers of the first m\$m\$ primes, i.e., k = p1α1⋯pmαm\$k=p_1^{\alpha_1}\cdots p_m^{\alpha_m}\$, where αm > 0\$\alpha_m>0\$.
We have that ab\$a^b\$ (b>1\$b>1\$) for some positive integer a\$a\$ if and only if b\$b\$ is a divisor of all exponents αj\$\alpha_j\$.
Thus, an integer k > 1\$k > 1\$ is a perfect power if and only if gcd(α1, ⋯, αm) ≠ 1\$\gcd(α_1, ⋯, α_m) ≠ 1\$.
How it works
µÆE;¬g/’µ#Ṫ Main link. No arguments.
µ Make the chain monadic, setting the left argument to 0.
µ# Find the first n integers k, greater or equal to 0, for which the
preceding chain returns a truthy value.
In the absence of CLAs, n is read implicitly from STDIN.
ÆE Compute the exponents of the prime factorization of k.
;¬ Append the logical NOT of k, i.e., 0 if k > 0 and 1 otherwise.
This maps 1 -> [0] and [0] -> [1].
g/ Reduce the list of exponents by GCD.
In particular, we achieved that 1 -> 0 and 0 -> 1.
’ Decrement; subtract 1 from the GCD.
This maps 1 to 0 (falsy) and all other integers to a truthy value.
Ṫ Tail; extract the last k.