FYI,

This does not do what you think it does:

open IN_FILE, $in_file || die "Couldn't open $in_file: $!\n";
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|| has a very high precedence. You probably want to use the lower-precedence 'or' operator here instead. Otherwise, your code above gets evaluated more like this:

open(IN_FILE, ($in_file || die "Couldn't open $in_file: $!\n"));
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Obviously not what you want.