• You are opening OUT for read, it shoud be open OUT, ">$file";
• The file name for OUT is the same as the flie name of IN, you will find that you clobber the contents on $file before you do anything

Open a tempory file, say "$file.tmp", write to that and then rename it to $file

Put

use strict;
use warnings;
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at the start of the script

Check you open the file handle

open IN, $file or die "Can't open $file: $!\n";
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