Perl Importing Variables From Calling Module

Question

I have a Perl module (Module.pm) that initializes a number of variables, some of which I'd like to import ($VAR2, $VAR3) into additional submodules that it might load during execution.

The way I'm currently setting up Module.pm is as follows:

package Module;

use warnings;
use strict;

use vars qw($SUBMODULES $VAR1 $VAR2 $VAR3);

require Exporter;
our @ISA = qw(Exporter);
our @EXPORT = qw($VAR2 $VAR3);

sub new {
    my ($package) = @_;
    my $self = {};
    bless ($self, $package);
    return $self;
}

sub SubModules1 {
    my $self = shift;
    if($SUBMODULES->{'1'}) { return $SUBMODULES->{'1'}; }

    # Load & cache submodule
    require Module::SubModule1;
    $SUBMODULES->{'1'} = Module::SubModule1->new(@_);    
    return $SUBMODULES->{'1'};
}

sub SubModules2 {
    my $self = shift;
    if($SUBMODULES->{'2'}) { return $SUBMODULES->{'2'}; }

    # Load & cache submodule
    require Module::SubModule2;
    $SUBMODULES->{'2'} = Module::SubModule2->new(@_);    
    return $SUBMODULES->{'2'};
}

Each submodule is structured as follows:

package Module::SubModule1;

use warnings;
use strict;
use Carp;

use vars qw();

sub new {
    my ($package) = @_;
    my $self = {};
    bless ($self, $package);
    return $self;
}

I want to be able to import the $VAR2 and $VAR3 variables into each of the submodules without having to reference them as $Module::VAR2 and $Module::VAR3. I noticed that the calling script is able to access both the variables that I have exported in Module.pm in the desired fashion but SubModule1.pm and SubModule2.pm still have to reference the variables as being from Module.pm.

I tried updating each submodule as follows which unfortunately didn't work I was hoping:

package Module::SubModule1;

use warnings;
use strict;
use Carp;

use vars qw($VAR2 $VAR3);

sub new {
    my ($package) = @_;
    my $self = {};
    bless ($self, $package);
    $VAR2 = $Module::VAR2;
    $VAR3 = $Module::VAR3;
    return $self;
}

Please let me know how I can successfully export $VAR2 and $VAR3 from Module.pm into each Submodule. Thanks in advance for your help!

Answer 1

In your submodules, are you forgetting to say

use Module;

? Calling use Module from another package (say Module::Submodule9) will try to run the Module::import method. Since you don't have that method, it will call the Exporter::import method, and that is where the magic that exports Module's variables into the Module::Submodule9 namespace will happen.

---

In your program there is only one Module namespace and only one instance of the (global) variable $Module::VAR2. Exporting creates aliases to this variable in other namespaces, so the same variable can be accessed in different ways. Try this in a separate script:

package Whatever;
use Module;
use strict;
use vars qw($VAR2);

$Module::VAR2 = 5;
print $Whatever::VAR2;    # should be 5.
$VAR2 = 14;               # same as $Whatever::VAR2 = 14
print $Module::VAR2;      # should be 14

Answer 2

Well there is the easy way:

In M.pm:

package M;

use strict;
use warnings;

#our is better than "use vars" for creating package variables
#it creates an alias to $M::foo named $foo in the current lexical scope 
our $foo = 5;

sub inM { print "$foo\n" }

1;

In M/S.pm

package M;

#creates an alias to $M::foo that will last for the entire scope,
#in this case the entire file
our $foo;

package M::S;

use strict;
use warnings;

sub inMS { print "$foo\n" }

1;

In the script:

#!/usr/bin/perl

use strict;
use warnings;

use M;
use M::S;

M::inM();
M::S::inMS();

But I would advise against this. Global variables are not a good practice, and sharing global variables between modules is even worse.