from perlvar:

HANDLE->autoflush(EXPR)
$OUTPUT_AUTOFLUSH # need to use English at the start of your module 
$|
    If set to nonzero, forces a flush right away and after every write
+ or print on the currently selected output channel. Default is 0 (reg
+ardless of whether the channel is really buffered by the system or no
+t; $| tells you only whether you've asked Perl explicitly to flush af
+ter each write). STDOUT will typically be line buffered if output is 
+to the terminal and block buffered otherwise. Setting this variable i
+s useful primarily when you are outputting to a pipe or socket, such 
+as when you are running a Perl program under rsh and want to see the 
+output as it's happening. This has no effect on input buffering. See 
+getc in the perlfunc manpage for that. (Mnemonic: when you want your 
+pipes to be piping hot.)
[download]

#see also http://perl.plover.com/FAQs/Buffering.html

use strict;
use warnings;

#prints, waits a second, prints
for (1..3) {
    sleep 1;
    print "$_\n";
}

#without the newline, waits the entire time and then prints the whole 
+thing.
for (1..3) {
    sleep 1;
    print "$_";
}

$| = 1;
#now it works
for (1..3) {
    sleep 1;
    print "$_";
}

#you might want to go back to the default behavior now.
$| = 0;
[download]

Considered (dvergin): Author requests this be listed on Tutorials page. What say you all?
Unconsidered (holli): Enough Keep Votes (Keep/Edit/Delete: 20/21/1)