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]

In reply to Re: Insert lines into specific stanza line by toolic
in thread Insert lines into specific stanza line by xjlittle

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