Is $MOD_PERL true or false? read(\*STDIN, $$buff, 13100, $offset); (as opposed to sysread) should return 13100 chars unless EOF or an error is encountered, but I don't know about the mod_perl method.

Even if read did act like sysread, there's no way it would return 10000 bytes given that 10000 is not even related to a power of two.

All signs point to some user-configured (or maybe even hardcoded) limit somewhere not related to read.