The problem is that send_email is not exported to the namespace of your calling script, so the subroutine is undefined. Two solutions: Use the fully-qualified name to call the sub, e.g. EmailSender::send_email( $username, $body ); or use Exporter to export the symbol to the calling namespace:

package EmailSender;

use base 'Exporter';

our @EXPORT_OK = ( 'send_email' );

...
1;
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And then in your calling script:

use EmailSender 'send_email';
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