[ic] customize email usertag

[Ayayu Infomation desk]

I would like to customize email usertag.
To use Japanese characters for e-mail subject, I need MIME::Base64
encode
in email usertag such as perl regular coding as follows.

use MIME::Base64();
$subject = "JAPANESE CHARACTERS";
chomp($subject);
$subject = "=?ISO-2022-JP?B?$subject?=";

But if I add line below in email usertag, Inertanl sever error occured.

$subject = MIME::Base64::encode($subject);

Is it impossible to use external library in usertag?
How should I do such a requirement?

Any suggestion?