use IO::All;    # install this module first

push @parts, Email::MIME->create(
    attributes  => {
        filename        => $attachments{doc_name},
        content_type    => $attachments{doc_mime_type},
        disposition     => 'attachment',
        encoding        => 'base64',
        name            => $attachments{doc_name}
    },
        body => io($attachments{doc_name})->all,  # add this line
);