use IO::Handle;
print "\n2\n"; # \n forces a flush, 2 printed immediately
print 1; # doesn't print, puts 1 in the buffer
sleep 1; # still nothing printed
STDOUT->autoflush(1); # buffered 1 printed
print "\n"; # prints immediately
sleep 1; 
print 1; # prints 1 immediately
sleep 1;
print "\n";