#!/usr/bin/perl -w
use strict;
    
open my $config, '+<', 'stanza' or die "stanza: $!\n";

my $ip = '12.34.56.78';
my $seen;
while (<$config>){ 
    if (m{^<stanza $ip>$}){
        $seen = 1;
    }
    if ($seen && m{</stanza>}){
        print "\tMy new data\n";
        $seen = 0;
    }

    print;
}
close $config;