I think the blackbox printer application does its own buffering. AFAIK, buffering is userspace stuff (done in libc or in other high level file handling library) and there's no way you can control it from the parent process. $|=1 sets Perl's idea of autoflush. After exec, Perl is gone. The exec'ed process inherits the bare POSIX STDOUT filehandle, not the stdio.h FILE struct, or the new perlio buffering filehandle object if you're using newer Perls etc.

Update: expanded a bit