Watch out for side effects with `use VERSION`

Item 83: Limit your distributions to the right platforms mentioned that use might invoke side effects. We didn’t get into the details in that Item though. As of Perl 5.10, use imports some feature that you might not want.

Merely specifying a Perl version prior to 5.10 does nothing other than check the version you specify against the interpreter version. If the version you specify is equal to or greater than the interpreter version, your program continues. If not, it dies:

use 5.008;  # needs perl5.008000 or later

This works with require too:

require 5.008;  # needs perl5.008000 or later

However, use is a compile-time function and require is a run-time function. By the time you hit that require, perl has already compiled your program up to that point or died trying as it ran into unknown features. Code may have already run, despite using an inappropriate version of perl. You want to impose your version restriction as soon as possible, so use is more appropriate since it happens earlier.

You might think that you can fix this with a BEGIN block which compiles and immediately runs the code so you get the ordering right. This gets the version check at compile time even though it’s a runtime statement:

BEGIN { require v5.10; }

In early versions of v5.10, this still imported new features, but this bug has been fixed. See BEGIN {require 5.011} imports features.

You should use at least v5.10.1 because it fixes various issues with smart match. That version doesn’t automatically import the new features if you use require. Either of these specify that version:

use v5.10.1;
BEGIN { require v5.10.1; }

use 5.010

With Perl 5.10, you get three side effects with use v5.10. Starting with that version, use-ing the version also pulls in the new features for that version. Obstensibly, that keeps programs designed for earlier versions breaking as newer perls add keywords, but it also tries to enforce the current philosophy of good programming on you.

Perl 5.10 introduces say, state, and given-which, which you import implicitly when you say use v5.10.1:

use v5.10.1;
say 'I can use Switch!';  # imported say()

given ($ARGV[0]) {        # imported given()
	when( defined ) { some_sub() }

sub some_sub {
	state $n = 0;         # imported state()
	say "$n: got a defined argument";

If you want to insist on v5.10 without its new features, perhaps because your code uses some of the same keywords already, you can unimport the side effects immediately with the new feature pragma:

use v5.10.1;     # implicit imports
no feature;    # take it right back again

# you're own version of say()
sub say {
	# something that you want to do

If you only want some of the new features, you can unimport the ones that you don’t want:

use v5.10.1;
no feature qw(say);   # leaves state() and given()

sub say {
	# something that you want to do

use 5.012

Perl 5.12 includes two more side effects for use VERSION. The unicode_strings feature treats all strings outside of bytes and locale scopes as Unicode strings. Additionally, use v5.12 automatically turns on strict:

use v5.12;
# now strictures are on

$foo = 1;   # compile-time error!

If, for some odd and dangerous reason you don’t want strict on by default, you can turn it off yourself, even though unimporting it doesn’t give you the warning that you’ve left the paved roads, you’ve just violated your rental car contract, and there’s a chainsaw massacrer waiting for you:

use v5.12;
no feature;
no strict;

my $foo = 1;   

$fo0++;  # sure, go ahead and make that error

A workaround to restrict perl versions

You can restrict the version more tightly by checking the value of the $] variable, just like the various examples you saw in Item 83:

	die "Unsupported version" 
		unless $] >= 5.010 and $] < 5.011

This has the added benefit of restricting the upper acceptable perl version. It works on older Perls too.

Things to remember

  • use VERSION imports new features since Perl 5.9.5.
  • BEGIN { require VERSION } still imports new features (fixed in later versions of v5.10 and v5.12)
  • Use no feature or no strict to unimport unwanted features.
  • Restrict the perl version with $].

One thought on “Watch out for side effects with `use VERSION`”

  1. There has been some talk on P5P of changing the way `use` works, so that it gets converted at compile time to `BEGIN { require; require feature; feature->import(“5.x”); }` or the roughly equivalent `BEGIN{ require; $^H{‘feature_unicode’} = q(1); $^H{‘feature_say’} = q(1); $^H{‘feature_state’} = q(1); $^H{‘feature_switch’} = q(1); }

    This is arguably how it should have been implemented to start with.

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