How can I share global values among different packages in Perl?

Question

Is there a standard way to code a module to hold global application parameters to be included in every other package? For instance: use Config;?

A simple package that only contains our variables? What about readonly variables?

Answer 1

There's already a standard Config module, so choose a different name.

Say you have MyConfig.pm with the following contents:

package MyConfig;

our $Foo = "bar";

our %Baz = (quux => "potrzebie");

1;

Then other modules might use it as in

#! /usr/bin/perl

use warnings;
use strict;

use MyConfig;

print "Foo = $MyConfig::Foo\n";

print $MyConfig::Baz{quux}, "\n";

If you don't want to fully qualify the names, then use the standard Exporter module instead.

Add three lines to MyConfig.pm:

package MyConfig;

require Exporter;
our @ISA = qw/ Exporter /;
our @EXPORT = qw/ $Foo %Baz /;

our $Foo = "bar";

our %Baz = (quux => "potrzebie");

1;

Now the full package name is no longer necessary:

#! /usr/bin/perl

use warnings;
use strict;

use MyConfig;

print "Foo = $Foo\n";

print $Baz{quux}, "\n";

You could add a read-only scalar to MyConfig.pm with

our $READONLY;
*READONLY = \42;

This is documented in perlmod.

After adding it to @MyConfig::EXPORT, you might try

$READONLY = 3;

in a different module, but you'll get

Modification of a read-only value attempted at ./program line 12.

As an alternative, you could declare in MyConfig.pm constants using the constant module and then export those.