The perforce plugin has a checkbox "Clean Workspace Before Each Build" to delete the workspace for a project. The maven2 plugin has an to "Use private Maven repository" which stores the maven2 .repository directory under the workspace.
Previously, checking the Clean Workspace option deleted everything in the workspace, including the .repository, which is useful to verify that transient maven dependencies are still available. An update to Hudson has altered the behavior so that Clean Workspace leaves the .repository directory in place. This appears related to the fix for http://issues.jenkins-ci.org/browse/JENKINS-7182. The fix should have included an option to specify whether cleaning the workspace includes cleaning the private maven repository.