Packages need to return true. If they don't it implies that some precondition for loading the package failed.

In most cases that just means putting 1; at the end of the package:

package Mail::Template;

use strict;
use base qw(Template::Base);

use Config;
use Mail::Sendmail qw(sendmail);
use Template::Base;
use Template::Config;

... # all the rest of the package

1; # The essential 'true'
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Some packages check their context in a BEGIN block and fail to load if it is not correct. That is fairly unusual, not least because the failure message is not wonderfully user friendly, as you have discovered.