While I can tell for sure where or what the problem is checking through your codes, I think you will do tell to heed the warning of this module's current maintainer MIME::Lite WAIT!
Secondly, this

  my $first_name  = $args{first_name}  || '';
  my $last_name   = $args{last_name}   || '';
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in your code, will perhaps be better written as

  my $first_name  = $args{first_name} // '';
  my $last_name   = $args{last_name}  // '';
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