"If, for some reason, you have to write to a file on an operating system that does no buffering, buffer yourself, and write large chunks."
This is odd. For this level of output, you're not looking so much at operating system buffering, but at the buffering your run-time environment provides. For Perl, this is given by the "normal" output functions (not sysread and syswrite). For C, this is e.g. the stdio.h functions. See K&R for details on how to implement putc and getc. It all happens in user space, not kernel space.
The vast majority of benefit from buffering comes from this application library level. In fact, the cited benchmarks say just that: switching off Perl's buffering (with $|=0) turns off this buffering; it doesn't do any hacking on obscure OS parameters. And doing it clobbers performance.
Juerd below is, of course, correct. You switch buffering off by making your output "piping hot": $|=1.
