Interpolating subject in 'mailto' links

Hello.

I want to put a 'reply' link under the message body. I tried:

<a href="mailto:
mailinglist(_at_)my-server(_dot_)com
?subject=Re:%20$SUBJECTNA:U$
&amp;in-reply-to=$MSGID:U$
&amp;references=$MSGID:U$
">
Reply</a>

(I added newlines just to make it easier to read.)

This works fine when the subject consists of ASCII letters, but if the 
subject is of the form:
=?iso-8859-1?B?...encoded text...?=

I get the literal string above (URL encoded) instead of the decoded text 
(URL encoded).
I tried to use "$SUBJECT:U$" instead of "$SUBJECTNA:U$", but it returns 
HTML markup.
Is there any way to solve this problem without modifying the Perl source?

Thanks in advance.