WebService::Xero 0.11 and earlier for Perl uses the rand() function as the default source of entropy, which is not cryptographically secure, for cryptographic functions. Specifically WebService::Xero uses the Data::Random library which specifically states that it is "Useful mostly for test programs". Data::Random uses the rand() function.
The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG's algorithm is not cryptographically strong.