An automatic labelling feature was officially introduced in the 2006.2 version of the Perforce Server. If your server was created from a 2006.2 or later version of p4d, then a "Revision" field will appear by default in the label spec form. The Revision field enables automatic labelling.
When upgrading an existing server to 2006.2, if the existing label spec template has been edited (and differs from the default template) then the edits will be preserved. As a consequence of editing, however, the Revision field will not be added. In this case, you will need to edit the label spec template to make the Revision field appear on new and existing labels.
p4 spec -o label > newspec
# edit newspec to add the following line to the Fields section:
312 Revision word 64 optional
p4 spec -i label < newspec