I presume the module is simply located in the same directory as the script rather than being "installed"?

Perl will usually look into the current directory (but not when using -T), but the current directory is not necessarily the one in which the script is located. Adding the following to your script should tell Perl where to look:

use Cwd            qw( realpath );
use File::Basename qw( dirname );

use lib dirname(realpath($0));
[download]