chomp(my @data = <DATA>);
while (my $confLine = <$in>) {
    if ($confLine =~ /ip4-address address="/) { # No need to backslash ".
        for my $ip (@data) {
            if ($confLine =~ /ip4-address address="\Q$ip/) {
                $confLine =~ s{/>}{update"/>};
            }
        }
    }
    print {$out} $confLine;
}

##</code><code>##

my $data_start = tell DATA;
while (my $confLine = <$in>) {
    if ($confLine =~ /ip4-address address="/) { # No need to backslash ".
        seek DATA, $data_start, 0;
        while (my $ip = <DATA>) {
            chomp $ip;
            if ($confLine =~ /ip4-address address="\Q$ip/) {
                $confLine =~ s{/>}{update"/>};
            }
        }
    }
    print {$out} $confLine;
}