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

##</code><code>##

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

##</code><code>##

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