I hope someone can enlighten me about a small problem I have.

I am writing a set of modules and want to use a global application configuration hash. Because some config depends on the system I run on, I thought I would use a BEGIN block to assign a config hash key based on the hostname. The BEGIN makes sure that this host dependent setup happens first.

However, in testing, I am seeing behaviour I am having trouble understanding.

If I assign a key inside the BEGIN to this global hash, I don't see it in the test program, unless no other keys are set in this global config hash.

Can someone explain this or direct me a decent explanation? I have looked at perlmod but still have trouble. This is Perl v5.10.0 on Debian Lenny.

Test program ttt.pl :

#!/usr/bin/perl -w

use Test::More tests => 1;
use ttt qw(get_key);
my ($a,$b,$c) = get_key();
is($a,1);
[download]

This gives a test FAILURE - we get "undef" for $a, not 1 :

package ttt;
use strict;
use warnings;
our @ISA         = qw(Exporter);
our @EXPORT      = (); 
our @EXPORT_OK   = qw(%config get_key);

our %config = ( 
    'key' => [undef,undef,undef],
);

BEGIN {
    $config{'key'} = [1,2,3];
}

sub get_key
{
    return @{$config{'key'}};
}

1;
[download]

If I replace the %config hash initialisation with just a declaration, it works. This passes ($a is 1):

package ttt;
use strict;
use warnings;
our @ISA         = qw(Exporter);
our @EXPORT      = (); 
our @EXPORT_OK   = qw(%config get_key);

our %config;

BEGIN {
    $config{'key'} = [1,2,3];
}

sub get_key
{
    return @{$config{'key'}};
}

1;
[download]

Trying to understand this failure, I thought it was because the %config assignment (to "undef") happens AFTER the "BEGIN" assigns (1,2,3) - so the key value is over-written with "undef".

However ...

If I assign ANY key/value in the declaration of %config, I get an error e.g.

This also fails :

package ttt;
use strict;
use warnings;
our @ISA         = qw(Exporter);
our @EXPORT      = ();
our @EXPORT_OK   = qw(%config get_key);

our %config = (
    'a' => 'test',
);                                                          

BEGIN {
    $config{'key'} = [1,2,3];
}

sub get_key
{
    return @{$config{'key'}};
}

1;
[download]

I get :

Can't use an undefined value as an ARRAY reference at ttt.pm line 21.
# Looks like your test died before it could output anything.
[download]

Every/Any config assignment has to happen inside the BEGIN. And I need to do it BEFORE the 'key' assignment i.e.

our %config;
BEGIN {
    %config = ( 'a' => 'test' );
    $config{'key'} = [1,2,3];
}
[download]

is OK

our %config;
BEGIN {
    $config{'key'} = [1,2,3];
    %config = ( 'a' => 'test' );
}
[download]

Gives an error :

Can't use an undefined value as an ARRAY reference at ttt.pm line 21.
[download]

What is happening here? Many thanks.

Alastair Sherringham