$mail{body} += encode_base64(<IN>); # addition won't work

##</code><code>##

$mail{body} = encode_base64(<IN>); # just assign it

##</code><code>##

$mail{body} .= encode_base64(<IN>); # just assign it

##</code><code>##

$mail{body} = <<EOD; <--- delimiter
...
$mail{body}
--$boundary
Content-Type: application/vnd.ms-excel; name="$file3"
Content-Disposition: attachment; filename="$file3"
Content-Transfer-Encoding: base64
Content-Length: $len

$mail{body}
$boundary--
END_OF_BODY          <-- wrong delimiter