Minor issue, which has been caused by my previous commits.
MacroStringHelper.substituteParameters() uses getEnvironment to get environment variables, which leads to recursive call with further exception.
+ P$CLIENT is a typo

Sep 12, 2013 4:29:40 AM hudson.plugins.perforce.PerforceSCM buildEnvVars
SEVERE: Can't substitute P$CLIENT
hudson.plugins.perforce.utils.ParameterSubstitutionException: <XBF_build_Perforce_ARCv2HS_SLAVE_${EXECUTOR_NUMBER}>: Found unresolved macro at 'XBF_build_Perforce_ARCv2HS_SLAVE_${EXECUTOR_NUMBER}'
at hudson.plugins.perforce.utils.MacroStringHelper.checkString(MacroStringHelper.java:96)
at hudson.plugins.perforce.utils.MacroStringHelper.substituteParameters(MacroStringHelper.java:79)
at hudson.plugins.perforce.PerforceSCM.getEffectiveClientName(PerforceSCM.java:1623)
at hudson.plugins.perforce.PerforceSCM.buildEnvVars(PerforceSCM.java:485)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:936)
at hudson.plugins.perforce.utils.MacroStringHelper.substituteParametersNoCheck(MacroStringHelper.java:139)
at hudson.plugins.perforce.utils.MacroStringHelper.substituteParameters(MacroStringHelper.java:78)
at hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:835)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1367)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:674)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:88)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:579)
at hudson.model.Run.execute(Run.java:1575)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:237)