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Const::Exporter(3) User Contributed Perl Documentation Const::Exporter(3)

Const::Exporter - Declare constants for export.

version v1.2.1

Define a constants module:

  package MyApp::Constants;

  our $zoo => 1234;

  use Const::Exporter

     tag_a => [                  # use MyApp::Constants /:tag_a/;
        'foo'  => 1,             # exports "foo"
        '$bar' => 2,             # exports "$bar"
        '@baz' => [qw/ a b c /], # exports "@baz"
        '%bo'  => { a => 1 },    # exports "%bo"

     tag_b => [                  # use MyApp::Constants /:tag_b/;
        'foo',                   # exports "foo" (same as from ":tag_a")
        '$zoo',                  # exports "$zoo" (as defined above)

  # `use Const::Exporter` can be specified multiple times

  use Const::Exporter

     tag_b => [                 # we can add symbols to ":tab_b"
        'moo' => $bar,          # exports "moo" (same value as "$bar")

     enums => [

       [qw/ goo gab gub /] => 0, # exports enumerated symbols, from 0..2


     default => [qw/ foo $bar /]; # exported by default

and use that module:

  package MyApp;

  use MyApp::Constants qw/ $zoo :tag_a /;


You may also import a predefined hash of constants for exporting dynamically:

 use Const::Exporter;

 my %myconstants = (
        'foo'  => 1,
        '$bar' => 2,
        '@baz' => [qw/ a b c /],
        '%bo'  => { a => 1 },

 # ... do stuff

      constants => [%myconstants],        # define constants for exporting
      default   => [ keys %myconstants ], # export everything in %myconstants by default

This module allows you to declare constants that can be exported to other modules.

To declare constants, simply group then into export tags:

  package MyApp::Constants;

  use Const::Exporter

    tag_a => [
       'foo' => 1,
       'bar' => 2,

    tag_b => [
       'baz' => 3,

    default => [

Constants in the "default" tag are exported by default (that is, they are added to the @EXPORTS array).

When a constant is already defined in a previous tag, then no value is specified for it. (For example, "bar" in "tab_b" above.) If you do give a value, Const::Exporter will assume it's another symbol.

Your module can include multiple calls to "use Const::Exporter", so that you can reference constants in other expressions, e.g.

  use Const::Exporter

    tag => [
        '$zero' => 0,

  use Const::Exporter

    tag => [
        '$one' => 1 + $zero,

or even something more complex:

  use Const::Exporter

     http_ports => [
        'HTTP'     => 80,
        'HTTP_ALT' => 8080,
        'HTTPS'    => 443,

  use Const::Exporter

     http_ports => [
        '@HTTP_PORTS' => [ HTTP, HTTP_ALT, HTTPS ],

Constants can include traditional constant symbols, as well as scalars, arrays or hashes.

Constants can include values defined elsewhere in the code, e.g.

  our $foo;

     $foo = calculate_value_for_constant();

  use Const::Exporter

    tag => [ '$foo' ];

Note that this will make the symbol read-only. You don't need to explicitly declare it as such.

Enumerated constants are also supported:

  use Const::Exporter

    tag => [

      [qw/ foo bar baz /] => 1,


will define the symbols "foo" (1), "bar" (2) and "baz" (3).

You can also specify a list of numbers, if you want to skip values:

  use Const::Exporter

    tag => [

      [qw/ foo bar baz /] => [1, 4],


will define the symbols "foo" (1), "bar" (4) and "baz" (5).

You can even specify string values:

  use Const::Exporter

    tag => [

      [qw/ foo bar baz /] => [qw/ feh meh neh /],


however, this is equivalent to

  use Const::Exporter

    tag => [
      'foo' => 'feh',
      'bar' => 'meh',
      'baz' => 'neh',

Objects are also supported,

   use Const::Exporter

    tag => [
      '$foo' => Something->new( 123 ),

By default, all symbols are exportable (in @EXPORT_OK.)

The ":default" tag is the same as not specifying any exports.

The ":all" tag exports all symbols.

This module requires Perl v5.10 or newer.

Pull requests to support older versions of Perl are welcome. See "SOURCE".

Const::Exporter is not intended for use with modules that also export functions.

There are workarounds that you can use, such as getting Const::Exporter to export your functions, or munging @EXPORT etc. separately, but these are not supported and changes in the future my break our code.

The following code is a syntax error, at least with some versions of Perl:

  use Const::Exporter

  =head2 a


    a => [ foo => 1 ],

  =head2 b


    b => [ bar => 2 ];

If you want to mix POD with your declarations, use multiple use lines, e.g.

  =head2 a


  use Const::Exporter
    a => [ foo => 1 ];

  =head2 b


  use Const::Exporter
    b => [ bar => 2 ];

See Exporter for a discussion of export tags.

This module only allows you to declare function symbol constants, akin to the constant module, without tags.
This module only allows you to declare function symbol constants, akin to the constant module, although you can specify tags.
This module only allows you to declare function symbol constants, akin to the constant module by defining functions that are only called as needed. The interface is rather complex.
This module will export all constants declared in the package's namespace.

The development version is on github at <> and may be cloned from <git://>

Please report any bugs or feature requests on the bugtracker website <>

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

Robert Rothenberg <>

  • Slaven Rezić <>
  • B. Estrade <>

This software is Copyright (c) 2014-2021 by Robert Rothenberg.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)
2021-09-22 perl v5.32.1

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