This will do it:
p4 copy //depot/project/dev/...@release_x_label //depot/project/release_x/...
The command tells Perforce to copy //depot/project/dev/... as of release_x_label to //depot/project/release_x/... You can run this command entirely server side using p4 copy -v. If you don't need a copy of the files locally it's much quicker.
You can do it in one go without needing a submit by using p4 populate
p4 populate //depot/project/dev/...@release_x_label //depot/project/release_x/...
I usually use copy though to make sure I don't have any typos. =)
You can do all of this with streams as well. It's a bit odd in that the revision specifier that you use to choose what to branch from gets put on the target path you're branching to.
p4 populate -rs //stream/dev2 //stream/dev2/...@1
This will populate dev2 from its parent as of changelist1.