Writing a "\n" to the disk does not force a flush. From perlvar:

STDOUT will typically be line buffered if output is to the terminal and block buffered otherwise.

Autoflush only applies if select() has been called to connect the handle to STDOUT and STDOUT is directed to the terminal.

In general everything you write to the disk is greedily buffered by the OS unless and until the OS runs out of buffer-cache, at which point everything slows to a crawl.