When updating the new data is written on the first or the last line of your log?

Do you have a my_log.old.gz file or my_log.1.gz file?

Any of the fields of your log is in any way a timestamp?

With this? are the lines unique? maybe you need more data:

while (@lines) {
     chomp;
     print if /\Q$pattern\E/;
   }
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