With Perforce, it’s not the end of the world if you forget to label a release before checking in more changes. As long as you know the last changelist number that belonged to the release, you can always make the label anyway. In fact, you can follow the instructions for Creating a Label, but alter them in the following way:
Ok immediately.Add/Replace only the following files.Other radio button below, select Changelist from the drop-down and enter the number. Files will be added to the label with the revision they had at the time that changelist was submitted.Ok to commit changes.