You want to generate numbers that are more random than Perl's random numbers. Limitations of your C library's random number generator seeds can sometimes cause problems. The sequence of pseudo-random numbers may repeat too soon for some applications.
Use a different random number generator, such as those provided by the Math::Random and Math::TrulyRandom modules from CPAN:
use Math::TrulyRandom; $random = truly_random_value( ); use Math::Random; $random = random_uniform( );
The Perl build process tries to find the best C-library routine to use for generating pseudo-random numbers, looking at rand(3), random(3), and drand48(3). (This can be changed manually at build time, however.) The standard library functions are getting pretty good, but some ancient implementations of the rand function return only 16-bit random numbers or have other algorithmic weaknesses, and may therefore not be sufficiently random for your purposes.
The Math::TrulyRandom module uses inadequacies of your system's timers to generate the random numbers. This takes a while, so it isn't useful for generating a lot of random numbers.
The Math::Random module uses the randlib library to generate random numbers. It also includes a wide range of related functions for generating random numbers according to specific distributions, such as binomial, poisson, and exponential.
The srand and rand functions in perlfunc(1) and Chapter 29 of Programming Perl; Recipe 2.6 and Recipe 2.7; the documentation for the CPAN modules Math::Random and Math::TrulyRandom