My guess is that you want to look at either IPC::Open3 or Expect.

With Expect, the new program will be on a tty, so it will be line buffered, however your stdout and stderr will be mixed together.

With IPC::Open3, the stdout and stderr will be seperated, but stdout won't be line buffered.

However, as it says in the IPC::Open3 docs, you can use IO::select to do non-blocking I/O on the resulting filehandles.