Original problem discussed in ticket JENKINS-27655 against perforce-plugin. After discussion and investigation, I found that a similar problem for the git-plugin was fixed in ssh-agent-plugin for JENKINS-12492, so I'm refiling this bug against ssh-agent-plugin.

Quick summary is that I have my ssh key credential loaded properly into ssh-agent-plugin, and while the key is available to the build steps themselves (such as a shell execution), the key is NOT available to the SCM/perforce step that comes before.