...it won't be buffered

Actually, STDOUT is line buffered by default when connected to a tty, i.e. - as zentara said - output will be written either upon encountering a newline, or when the buffer has filled up (or when the program terminates, and the buffer gets flushed).

Try playing with something like

for (1..3000){
    print "$_ ";
    sleep 5 unless $_ % 1000;
}
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