Indeed. I concur that there is nothing wrong with your sed text replacement. Your code is printing the value of $WW before it is modified, but not after.

Additional comments:

Use 'my' instead of 'local' in your array assignment. It is the correct localization for what you are doing.

Text::CSV_XS does comma separated value parsing much better than you can. Since your values have text in them, they could also have embedded sequences that will throw off your simple parser.

You don't have to "QUOTE" singleton right value scalars.

My preference is to make flag values integers so that you can say something like this:

my $kill_flag++ if ($formate_date1 > $formate_date2);
if ($kill_flag) { }
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My preference is to mark all variables using 'my' variable scoping upon their first use. It localizes the value and it can protect you in case you make this a required external code block or move it to a CLASS.