Just detect the closing stanza tag, then add lines before it:

use strict;
use warnings;

while (<DATA>) {
    print "extra line\n" if m{</stanza>};
    print;
}

__DATA__
<stanza 12.34.56.78>
    sl1  this is my line
    sl2  justanotherline
</stanza>
[download]

prints out:

<stanza 12.34.56.78>
    sl1  this is my line
    sl2  justanotherline
extra line
</stanza>
[download]