First I'll comment on the move from Perl to MJC.  We did some initial testing of our current automation projects under the MJC and the results were largely disappointing.  While the step-level performance was significantly improved (30-40% in our tests) there are two major functionality showstoppers for us.  1. When executing a job, the steps are only displayed on the job page as each one is executed.  So the "pause step" functionality for is effectively useless.  2. We do a lot of dynamic step generation and then use the ".load" dot command to execute the steps.  This functionality simply doesn't work. It always fails to load any steps.

We've discontinued all efforts to test/evaluate the MJC and will stick to the Perl engine.

For the Assign Hidden variable problem in the Perl engine you will be fine if you never add or update a Assign Hidden variable (how likely is that?!).  Once you do that the value becomes encrypted and will no longer work with your existing agents and project implementations.  And the new Assign Hidden functionality requires you to pass the variable as an argument to your scripts/commands as the value itself remains encrypted until the agent decrypts it before calling your scripts/command.  So your script can't use the value from the ENV since it's encrypted.  And even that functionality doesn't work unless the Assign Hidden variable is in an Environment Group attached to the project.  I.e., if the Environment Group where the Assign Hidden variable is is attached to a step it doesn't work.  Rational provided us a test fix agent that resolves that bug.

We've had other regression issues with release 8 in the Perl engine as well and it's been a struggle with numerous work-arounds required to keep our automated processes running.  We've learned our lesson that if we really want to upgrade in the future we will have to perform significant regression testing of our own to ensure product stability.