$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);

##</code><code>##

select((select(STDERR), $| = 1)[0])

##</code><code>##

use IO::Handle;
OUTPUT_HANDLE->autoflush(1);

##</code><code>##

use FileHandle;
STDOUT->autoflush(1);

##</code><code>##

use IO::Handle;
autoflush ONE_HANDLE 1;     # unbuffer for clarity
autoflush ANOTHER_HANDLE 0; # buffer this for speed