I'm not exactly sure what you are asking. You mentioned a problem with command initialization time, but then you're asking for a suggestion on CPAN modules.
What exactly are you spawning with the Proc::Background? A shell? Then you want to send more commands to it? I'm curious why you wouldn't just fork a new Proc::Background process with a new command. Perhaps if you gave a solid example of what you are trying to accomplish?
Perhaps I'm just not understanding your question...