package mdlno;
use strict;
use warnings;
use feature 'say';

my $param = shift;

say main() unless caller();

sub main {
    $param //= 'World';
    return 'Hello ' . $param;
}

##</code><code>##

use strict;
use warnings;
use Test::More tests => 2;
use FindBin qw( $RealBin );
use lib $RealBin;
use mdlno;

my $rtn = mdlno->main();
is $rtn, 'Hello World', 'Works without parameter';
$mdlno::param = 'and goodbye';
$rtn = mdlno->main();
is $rtn, 'Hello and goodbye', 'Works with parameter';