This error shows up, but not consistently if the operating system is upgraded to use perl 5.16 or higher. (The new perl has an asynchronous loading pattern which is not allowed for in the original code.) The pull requests webwork2: #465 and pg #148 address this issue. Both pull requests need to be applied to the develop branch at the same time since files are changed in both the webwork2 and pg directories.
After further testing these pull requests will become part of the develop release.