** Using WeBWorK **

The problem with what you've added is that it did exactly what you feared.
You have replaced the complete list of permissions for the course with a hash containing just a single permission.
What you want is something like the following instead:
$permissionLevels{send_mail} = 'ta';
IIRC, the 'send_mail' permission refers to being able to send email from within a pg problem (for essay questions, etc.). Are you trying to change who can use the "email professor" button? If so, you should be changing the 'submit_feedback' permission instead (which can be done from the Course Configuration link on the web interface).