<code></code> tags are the preferred way to post formatted plain text here.

The input record separator needs to be set to something, and 8kb is a reasonable value. It shouldn't be "\n" because you might not have any. But you can use it for your new problem:

$/ = '1397';  # You don't have to specify the hex numbers; it's all th
+e same to Perl
while (<>) {
    # 1397 appears at the end of the record, if at all
    # so an anchored search-and-replace handles it
    s/1397$/\0\0\0\0/;
}
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