Well, to be even more nitpicky, is the question the "highest number" (i.e. largest number that can be represented in perl), or the "period" of the pseudo-random number generator (i.e. the number of random numbers that can be generated before the cycle repeats).

Generally, the second is the more interesting value. And it's certainly possible to get higher periodicities in Perl than what comes with the built-in rand. Based on its release activity and documentation, I'd recommend anyone needing high-quality pseudo-randoms look at Math::Random::MT::Auto, which can be used as a drop-in replacement for rand.

use Math::Random::MT::Auto 'rand';
[download]

It offers a periodicity of 2^19937 - 1 using the Mersenne twister algorithm. Among other useful features is the irand function to directly generate random integers. So if your Perl is compiled for 64-bit integers, you can generate 64-bit pseudo-random numbers.