Here is the command to integrate or merge two branches together in Perforce.
p4 integrate -c CL -i -d -t -b BRANCH_SPEC -s SOURCE_DIRECTORY@SOURCE_CL|
If you want to know what each argument stands for, then I recommend that you read the help information by running this command:
p4 help integrate|
After integrating, all the merged files will be checkedout or open in your CL, so you probably need to resolve merge issues and conflicts before being able to submit your changes.
by Phil for Humanity