The Perl Advent Calendar needs more articles for 2022. Submit your idea today!


exact - Perl pseudo pragma to enable strict, warnings, features, mro, filehandle methods


version 1.19


Instead of this:

    use strict;
    use warnings;
    use utf8;
    use open ':std', ':utf8';
    use feature ':5.32';
    use feature qw( signatures refaliasing bitwise isa );
    no feature 'indirect';
    use mro 'c3';
    use IO::File;
    use IO::Handle;
    use namespace::autoclean;
    use Carp qw( croak carp confess cluck );
    use Syntax::Keyword::Try;

    no warnings "experimental::signatures";
    no warnings "experimental::refaliasing";
    no warnings "experimental::bitwise";

Type this:

    use exact;

Or for finer control, add some trailing modifiers like a line of the following:

    use exact -noexperiments, -fc, -signatures;
    use exact 5.16, -nostrict, -nowarnings, -noc3, -noutf8, -noautoclean;
    use exact '5.20';


exact is a Perl pseudo pragma to enable strict, warnings, features, mro, and filehandle methods along with a lot of other things, plus allow for easy extension via exact::* classes. The goal is to reduce header boilerplate, assuming defaults that seem to make sense but allowing overrides easily.

By default, exact will:

  • enable strictures (version 2)

  • activate the latest feature bundle supported by the current Perl version

  • activate all experimental features and switch off experimental warnings

  • activate the isa feature (if Perl version is 5.32 or greater)

  • deactivate the indirect feature

  • set C3 style of mro

  • use utf8 in the source code context and set STDIN, STROUT, and STRERR to handle UTF8

  • enable methods on filehandles

  • import Carp's 4 methods

  • cause Syntax::Keyword::Try to import its methods


exact supports the following import flags:


This skips turning on the strict pragma.


This skips turning on the warnings pragma.


This skips turning on UTF8 in the source code context. Also skips setting STDIN, STDOUT, and STDERR to expect UTF8.


This skips setting C3 mro.


Normally, exact will look at your current version and find the highest supported feature bundle and enable it. Applying nobundle causes this behavior to be skipped. You can still explicitly set bundles yourself.


This skips enabling all features currently labled experimental by feature.


Normally, exact will disable experimental warnings. This skips that disabling step.


This skips using namespace::autoclean.


This skips importing the 4 Carp methods: croak, carp, confess, cluck.


This skips importing the functionality of Syntax::Keyword::Try.


If you want to use Try::Tiny instead of Syntax::Keyword::Try, this is how. Note that if you specify both trytiny and notry, the latter will win.


The isa feature is activated by default if the Perl version is 5.32 or greater. If you want not that, specify noisa.


You can always provide a list of explicit features and bundles from feature. If provided, these will be enabled regardless of the other import flags set.

    use exact -noexperiments, -fc, -signatures;

Bundles provided can be exactly like those described in feature or in a variety of obvious forms:

  • :5.26

  • 5.26

  • v5.26

  • 26

Note that bundles are exactly the same as what's in feature, so for any feature not part of a version bundle in feature, you won't pick up that feature with a bundle unless you explicitly declare the feature.

The exception to this is isa, which is available in Perl 5.32 and greater but not included in the 5.32 bundle. However, isa is explicitly included if your Perl version is 5.32 or greater unless you specify noisa.

Note also that the indirect feature is unimported by default, which is counter to the non-exact default way, which is to import it. You can deunimport indirect by explicitly specifying indirect.


It's possible to write extensions or plugins for exact to provide context-specific behavior, provided you are using Perl version 5.14 or newer. To activate these extensions, you need to provide their named suffix as a parameter to the use of exact.

    # will load "exact" and "exact::class";
    use exact -class;

    # will load "exact" and "exact::role" and turn off UTF8 features;
    use exact -role, -noutf8;

It's possible to provide parameters to the import method of the extension.

    # will load "exact" and "exact::answer" and pass "42" to the import method
    use exact 'answer(42)';

Writing Extensions

An extension may but is not required to have an import method. If such a method does exist, it will be passed: the package name, the name of the caller of exact, and any parameters passed.

    package exact::example;
    use exact;

    sub import {
        my ( $self, $caller, $params ) = @_;
        exact->monkey_patch( $caller, 'example' => \&example );

    sub example {
        say 42;



You can use exact to setup inheritance as follows:

    use exact 'SomeModule', 'SomeOtherModule';

This is roughly equivalent to:

    use exact;
    use parent 'SomeModule', 'SomeOtherModule';

See also the no_parent method.



Monkey patch functions into a given package.

    exact->monkey_patch( 'PackageName', add => sub { return $_[0] + $_[1] } );
        one   => sub { return 1 },
        two   => sub { return 2 },
        three => sub { return 3 },


This method will add a given parent to the @ISA of a given child.

    exact->add_isa( 'SuperClassParent', 'SubClassChild' );


Normally, if you specify a parent, it'll be added as a parent by inclusion in @INC. If you don't want to skip @INC inclusion, you can call no_parent in the import of the module being specified as a parent.

    sub import {


There may be a situation where you need an included parent to be listed last in @INC (at least relative to other parents). Normally, you'd do this by putting the name last in the list of modules. However, if for some reason you can't do that, you can call late_parent from the import of the parent that should be delayed in @INC inclusion.

    sub import {


This method performs work similar to using Exporter's @EXPORT, but only for methods. For a given method within your package, it will be exported to the namespace that uses your package.

    exact->export( 'method', 'other_method' );


This method performs work similar to using Exporter's @EXPORT_OK, but only for methods. For a given method within your package, it will be exported to the namespace that uses your package.

    exact->exportable( 'method', 'other_method' );

It's possible to provide hashrefs as input to this method, and doing so provides the means to setup groups of methods a consuming namespace can import.

            ':stuff' => [ qw( method other_method ) ],
            ':all'   => [ qw( method other_method some_additional_method ) ],

In the consuming namespace, you can then write:

    use YourPackage ':stuff'; # imports both "method" and "other_method"


You can look for additional information at:


Gryphon Shafer <>


This software is Copyright (c) 2017-2050 by Gryphon Shafer.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)