One weakness of Perforce is that it assumes that network access to the server is always available. Most state, history, and metadata is saved on the server, and there is no provision for replicating the server like there is with CVS/CVSup. It is possible to run a proxy server, but it only provides very limited utility for offline operation.
The best way to work offline is to make sure that your client has no open files and is fully synced before going offline. Then when editing a file, manually change the permissions to read-write. When you get back online, run the commands listed in the Section 12 to automatically identify files that have been edited, added, and removed. It is quite common to be surprised by Perforce overwriting a locally changed file that was not opened for edit, so be extra vigilant with this.