"Perl Idioms Explained" explained

This is the first post in a series of posts that aim to explain the workings and reasoning behind common perl idioms. Each post will be relatively short but hopefully informative enough to sufficiently illuminate each idiom.

Perl Idioms Explained - $|++

use strict;
use warnings;

$|++;

use Some::Module 'process_stuff';

print '.' while process_stuff();
[download]

It is not entirely uncommon to see this rather mysterious combination of characters following the usual pragma declarations - $|++. The most perfunctory glance will reveal that the post-increment operator is used, and a further look will show that said operator is working on the $|. Browsing through man perlvar we see that $| is defined like so

autoflush HANDLE EXPR
$OUTPUT_AUTOFLUSH
$|

If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel. Default is 0 (regardless of whether the channel is really buffered by the system or not; "$|" tells you only whether you've asked Perl explicitly to flush after each write). STDOUT will typically be line buffered if output is to the terminal and block buffered otherwise. Setting this variable is 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 the getc entry in the perlfunc manpage for that. (Mnemonic: when you want your pipes to be piping hot.)

So the the post-increment on the $| sets it to 1 (sidenote - its value, magically, can only ever be 0 or 1) which will turn off buffering for STDOUT. The implications for turning off buffering are far reaching so if you'd like to know more about it then see Dominus' cracking Suffering from Buffering. For a simple demonstration of this idiom in action and the effects of output buffering try running the following piece of code

print "printing dots with buffering\n";
select undef, undef, undef, 0.25 or print '.'
  for 1 .. 10;
print "\nflush forced\n";

$|++;
print "printing dots without buffering\n";
select undef, undef, undef, 0.25 or print '.'
  for 1 .. 10;
print "\nflush forced\n";
[download]

When that is run you should notice that with buffering the dots aren't displayed until the flush is forced with a \n, whereas without buffering the dots were displayed immediately as the output wasn't being held in an output buffer.

Summary

The $|++ idiom will turn buffering off for when we want to see our output, and we want to see it now!

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broquaint

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