In this case, because STDOUT was made "hot" by setting $| to 1, perl flushed its buffer to the system every time "\n" was written.

That's not quite true. By setting $| to 1, Perl flushes the buffer every time anything is written. There are three states, not two:

$ strace perl -e'$|=0; print for "a", "b\n", "c\n"' >/dev/tty
write(1, "ab\n", 3)                     = 3
write(1, "c\n", 2)                      = 2

$ strace perl -e'$|=0; print for "a", "b\n", "c\n"' >/dev/null
write(1, "ab\nc\n", 5)                  = 5

$ strace perl -e'$|=1; print for "a", "b\n", "c\n"'
write(1, "a", 1)                        = 1
write(1, "b\n", 2)                      = 2
write(1, "c\n", 2)                      = 2
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