...
use Config qw(getConfig);
...
my $foo = getConfig('bar');
...

##</code><code>##

package Config;

my $initialised = 0;

sub _initialise {
    return if $initialised;
    ... # read config file
    $initialised++;
}

_initialise();

sub getConfig{...}
...
1;

##</code><code>##

package Config;

our $initialised = 0;

sub _initialise {
    ... # as before
}

BEGIN {
    _initialise();
}
...
1;

##</code><code>##

...
use_ok('Config');
is($Config::initialised, 1); # initialisation completed OK

##</code><code>##

t/Config....Name "Config::var" used only once: possible typo at t/Config.t line 9.
#     Failed test (t/Config.t at line 9)
#          got: '0'
#     expected: '1'
# Looks like you failed 1 tests of 2.
t/Config....dubious
        Test returned status 1 (wstat 256, 0x100)
DIED. FAILED test 2
        Failed 1/2 tests, 50.00% okay
Failed Test Stat Wstat Total Fail  Failed  List of Failed
-------------------------------------------------------------------------------
t/Config.t     1   256     2    1  50.00%  2
Failed 1/1 test scripts, 0.00% okay. 1/2 subtests failed, 50.00% okay.
make: *** [test_dynamic] Error 2

##</code><code>##

package Config;

sub _initialise {
    ... # as before
}

BEGIN {
    our $initialised = 0;

    _initialise();
}

1;