Take a gander at Devel::Command::Tdump for an example of using the typeahead buffer. Debug::Client seems to be a client to talk to the debugger port.

I don't see any examples of using the filehandle stack - you probably have to do that one yourself if it turns out to be the solution you need - but the basic setup that Devel::Command::Tdump uses, of setting things up in .perldb, is probably the right point to start from; open a pipe to your client program in the BEGIN block, and push it onto @DB::cmdfhs. The debugger will read from it until it returns EOF or a 'q'commmand is seen.