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Manual Reference Pages  -  STRICTURES (3)

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strictures - turn on strict and make most warnings fatal



  use strictures 2;

is equivalent to

  use strict;
  use warnings FATAL => all;
  use warnings NONFATAL => qw(
  no warnings once;

except when called from a file which matches:

  (caller)[1] =~ /^(?:t|xt|lib|blib)[\\\/]/

and when either .git, .svn, .hg, or .bzr is present in the current directory (with the intention of only forcing extra tests on the author side) — or when .git, .svn, .hg, or .bzr is present two directories up along with dist.ini (which would indicate we are in a dzil test operation, via Dist::Zilla) — or when the PERL_STRICTURES_EXTRA environment variable is set, in which case it also does the equivalent of

  no indirect fatal;
  no multidimensional;
  no bareword::filehandles;

Note that PERL_STRICTURES_EXTRA may at some point add even more tests, with only a minor version increase, but any changes to the effect of use strictures in normal mode will involve a major version bump.

If any of the extra testing modules are not present, strictures will complain loudly, once, via warn(), and then shut up. But you really should consider installing them, they’re all great anti-footgun tools.


I’ve been writing the equivalent of this module at the top of my code for about a year now. I figured it was time to make it shorter.

Things like the importer in use Moose don’t help me because they turn warnings on but don’t make them fatal — which from my point of view is useless because I want an exception to tell me my code isn’t warnings-clean.

Any time I see a warning from my code, that indicates a mistake.

Any time my code encounters a mistake, I want a crash — not spew to STDERR and then unknown (and probably undesired) subsequent behaviour.

I also want to ensure that obvious coding mistakes, like indirect object syntax (and not so obvious mistakes that cause things to accidentally compile as such) get caught, but not at the cost of an XS dependency and not at the cost of blowing things up on another machine.

Therefore, strictures turns on additional checking, but only when it thinks it’s running in a test file in a VCS checkout — although if this causes undesired behaviour this can be overridden by setting the PERL_STRICTURES_EXTRA environment variable.

If additional useful author side checks come to mind, I’ll add them to the PERL_STRICTURES_EXTRA code path only — this will result in a minor version increase (e.g. 1.000000 to 1.001000 (1.1.0) or similar). Any fixes only to the mechanism of this code will result in a sub-version increase (e.g. 1.000000 to 1.000001 (1.0.1)).


strictures does not enable fatal warnings for all categories.
exec Includes a warning that can cause your program to continue running unintentionally after an internal fork. Not safe to fatalize.
recursion Infinite recursion will end up overflowing the stack eventually anyway.
internal Triggers deep within perl, in places that are not safe to trap.
malloc Triggers deep within perl, in places that are not safe to trap.
newline Includes a warning for using stat on a valid but suspect filename, ending in a newline.
experimental Experimental features are used intentionally.
deprecated Deprecations will inherently be added to in the future in unexpected ways, so making them fatal won’t be reliable.
portable Doesn’t indicate an actual problem with the program, only that it may not behave properly if run on a different machine.
once Can’t be fatalized. Also triggers very inconsistently, so we just disable it.


Depending on the version of strictures requested, different warnings will be enabled. If no specific version is requested, the current version’s behavior will be used. Versions can be requested using perl’s standard mechanism:

  use strictures 2;

Or, by passing in a version option:

  use strictures version => 2;


Equivalent to:

  use strict;
  use warnings FATAL => all;
  use warnings NONFATAL => qw(
  no warnings once;

  # and if in dev mode:
  no indirect fatal;
  no multidimensional;
  no bareword::filehandles;

Additionally, any warnings created by modules using warnings::register or warnings::register_categories() will not be fatalized.


Equivalent to:

  use strict;
  use warnings FATAL => all;
  # and if in dev mode:
  no indirect fatal;
  no multidimensional;
  no bareword::filehandles;



This method does the setup work described above in DESCRIPTION. Optionally accepts a version option to request a specific version’s behavior.


This method traps the strictures->VERSION(1) call produced by a use line with a version number on it and does the version check.


Every so often, somebody complains that they’re deploying via git pull and that they don’t want strictures to enable itself in this case — and that setting PERL_STRICTURES_EXTRA to 0 isn’t acceptable (additional ways to disable extra testing would be welcome but the discussion never seems to get that far).

In order to allow us to skip a couple of stages and get straight to a productive conversation, here’s my current rationale for turning the extra testing on via a heuristic:

The extra testing is all stuff that only ever blows up at compile time; this is intentional. So the oft-raised concern that it’s different code being tested is only sort of the case — none of the modules involved affect the final optree to my knowledge, so the author gets some additional compile time crashes which he/she then fixes, and the rest of the testing is completely valid for all environments.

The point of the extra testing — especially no indirect — is to catch mistakes that newbie users won’t even realise are mistakes without help. For example,

  foo { ... };

where foo is an & prototyped sub that you forgot to import — this is pernicious to track down since all seems fine until it gets called and you get a crash. Worse still, you can fail to have imported it due to a circular require, at which point you have a load order dependent bug which I’ve seen before now only show up in production due to tiny differences between the production and the development environment. I wrote <> to explain this particular problem before strictures itself existed.

As such, in my experience so far strictures’ extra testing has avoided production versus development differences, not caused them.

Additionally, strictures’ policy is very much try and provide as much protection as possible for newbies — who won’t think about whether there’s an option to turn on or not — so having only the environment variable is not sufficient to achieve that (I get to explain that you need to add use strict at least once a week on freenode #perl — newbies sometimes completely skip steps because they don’t understand that that step is important).

I make no claims that the heuristic is perfect — it’s already been evolved significantly over time, especially for 1.004 where we changed things to ensure it only fires on files in your checkout (rather than strictures-using modules you happened to have installed, which was just silly). However, I hope the above clarifies why a heuristic approach is not only necessary but desirable from a point of view of providing new users with as much safety as possible, and will allow any future discussion on the subject to focus on how do we minimise annoyance to people deploying from checkouts intentionally.


o indirect
o multidimensional
o bareword::filehandles


    IRC channel #toolchain

(or bug ’mst’ in query on there or freenode)

    Git repository

Gitweb is on and the clone URL is:

  git clone git://

The web interface to the repository is at:


mst - Matt S. Trout (cpan:MSTROUT) <>


Karen Etheridge (cpan:ETHER) <>

Mithaldu - Christian Walde (cpan:MITHALDU) <>

haarg - Graham Knop (cpan:HAARG) <>


Copyright (c) 2010 the strictures AUTHOR and CONTRIBUTORS as listed above.


This library is free software and may be distributed under the same terms as perl itself.
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perl v5.20.3 STRICTURES (3) 2015-11-04

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