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foreach my $file (readdir D) {
   my ($filebase, $dirname, $ext) = fileparse($file, '\..*');
   my $csvFile = "$opts{d}\\$filebase.csv";
   open FILE_OUT, ">$csvFile" or die ...;

for my $file ( grep /\.txt$/, readdir D )

for my $file (<$opts{d}/*.txt>) # use a file glob