Text entry should trim whitespace when whitespace results in a configuration error. Example:
Setup > Path to p4 executable

add a space to the end of the entry, e.g.,
"/usr/bin/p4 "

Result: java.io.IOException: Cannot run program "/usr/bin/p4 "

Deleting the whitespace at the end of the command fixes the problem.

The erroneous space character was the result of a copy/paste operation. It would simplify things to have the perforce plugin trim the whitespace.

Console output:

=====================================
Started by user anonymous
Using master perforce client: client.name
[workspace] $ "/usr/bin/p4 " workspace -o client.name
Caught exception communicating with perforce. Could not run perforce command.com.tek42.perforce.PerforceException: Could not run perforce command.
at hudson.plugins.perforce.HudsonP4Executor.exec(HudsonP4Executor.java:83)
at com.tek42.perforce.parse.AbstractPerforceTemplate.getPerforceResponse(AbstractPerforceTemplate.java:289)
at com.tek42.perforce.parse.Workspaces.getWorkspace(Workspaces.java:53)
at hudson.plugins.perforce.PerforceSCM.getPerforceWorkspace(PerforceSCM.java:951)
at hudson.plugins.perforce.PerforceSCM.checkout(PerforceSCM.java:499)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1054)
at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
at hudson.model.Run.run(Run.java:1248)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)
Caused by: java.io.IOException: Cannot run program "/usr/bin/p4 " (in directory "/var/lib/hudson/jobs/ProgressiveProxy/workspace"): java.io.IOException: error=2, No such file or directory
at java.lang.ProcessBuilder.start(ProcessBuilder.java:475)
at hudson.Proc$LocalProc.<init>(Proc.java:192)
at hudson.Proc$LocalProc.<init>(Proc.java:164)
at hudson.Launcher$LocalLauncher.launch(Launcher.java:638)
at hudson.Launcher$ProcStarter.start(Launcher.java:273)
at hudson.plugins.perforce.HudsonP4Executor.exec(HudsonP4Executor.java:74)
... 11 more
Caused by: java.io.IOException: java.io.IOException: error=2, No such file or directory
at java.lang.UNIXProcess.<init>(UNIXProcess.java:164)
at java.lang.ProcessImpl.start(ProcessImpl.java:81)
at java.lang.ProcessBuilder.start(ProcessBuilder.java:468)
... 16 more
Finished: FAILURE