perl -Mbigint -E "use Math::Prime::Util::GMP qw(factor); say 0+(factor(600851475143600851475143600851475143))[
-1]"

999999000001

# ~1 second to get the largest factor of:
# 6008514751436008514751436008514751430085147514300851475143008514751430085147514300851475143

834789126661809633627036527926883609177018602883402075695455196267