I would just add the possibility to take yet optimized code from CPAN: Math::Prime::Util (or Math::Prime::Util::GMP directly for big numbers.. but see also..)

From examples:

# Project Euler, problem 3 (Largest prime factor)

use Math::Prime::Util qw/factor/;
use bigint;  # Only necessary for 32-bit machines.
say 0+(factor(600851475143))[-1]
[download]

PS wow it returns in no time!:

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

999999000001

# ~1 second to get the largest factor of:
# 60085147514360085147514360085147514300851475143008514751430085147514
+30085147514300851475143

834789126661809633627036527926883609177018602883402075695455196267
[download]

L*