while (@array)
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This will not do what you expect it to do (specifically, the loop will not terminate.) Please fix your code before posting it.

Based on the rest of your code, I assume that your stanza is contained in a file, and that your data is laid out just the way you specify - tags, etc. each on a line of its own. In that case, the answer is fairly simple:

#!/usr/bin/perl -w
use strict;

open my $config, '+<', 'stanza' or die "stanza: $!\n";
my @all = <$config>;
seek $config, 0, 0;
splice @all, -1, 0, "\tMy new data\n";
print $config @all;
close $config;
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This will insert the "\tMy new data\n" string just before the closing stanza.

In case you need to select one of multiple stanzas by IP, here's a different version:

#!/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;
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