Close the files you created before reading them

close $clss_out{$_} for keys %clss_out;
csv_xlsx();
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Also, consider deleting the csv file individually rather that using glob

sub csv_xlsx{
  my $dir = "C:\\Temp\\Data";
  my @csv_files = glob "$dir/*.csv";
  foreach my $csv (@csv_files) {
    (my $xlsx = $csv) =~ s/csv/xlsx/i;
    print "Creating $xlsx ";
    my $workbook  = Excel::Writer::XLSX->new($xlsx);
    my $worksheet = $workbook->add_worksheet();

    my $row = 0;
    open my $fh, '<', $csv or die "Can't open '$csv' : $!";
    while (<$fh>) {
      my @list = split /,/;
      $worksheet->write_row( $row, 0, \@list );
      ++$row;
    }
    print "$row rows written\n";
    $workbook->close();
    close $fh or die "Can't close '$csv' ; $!";
    unlink $csv or warn $! ;
  }
}
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Update : Why are you using this ?

my $xlsx_WSD = ( "C:\\Temp\\Data\\data.xlsx"),, 1;

instead of simply

my $xlsx_WSD = "C:\\Temp\\Data\\data.xlsx";
poj