In addition to your changes and/or as an replacement of your changes ...
do this:

sub proc_test
delete all instances of $isbn

    for (1 .. 50) {
          $form{'TITLE'} = 'Test Item';
          $form{'ISBN'} = '0123456789';
          $form{'DESC'} = 'Test Description';
          $form{'TITLE'} .= " $_";
          $form{'DESC'} .= " $_";
          $close_time = (time + 120 + ($_ * 60));
          #$close_time = (time + (86400 * 60) + ($_ * 60)); # Author's line
     
         $item_number = write_item_file(1,
               $form{'CATEGORY'},
               $form{'TITLE'},
               $form{'ISBN'},
               $form{'RESERVE'},
               $form{'INC'},
               $form{'DESC'},
               $form{'IMAGE0'},
               $start_time,
               $close_time,
               $clicks,
               $seller_data
          );

################################
sub disp_list
  'isbn'  => q|$isbn eq $form{'searchstring'}|,

################################
sub disp_user_auction

   my (
        $query, $sub, $href, $alias, $password, $item, $cat, $title, $reserve, $image,
        $close_time, $high_bid, $bidder, $action, $img_info, $row, $rowcolor, $tbl_val,
        $record, $seller, $alias_to_rate, $isbn,
        @firstbid, @lastbid, @closetime, @bids, @ratings, @record
   );

   
 (
     $title,
     $isbn,
     $reserve,
     undef,
     undef,
     $image,
     undef,
     $close_time,
     undef,
     @bids
 ) = @{$sub->($cat, $item)};

################################
sub get_random_data

   my (
        $item, $title, $image, $start_time,
        $close_time, $the_bid, @bids
   );


  (
     $title,
     undef,
     undef,
     undef,
     undef,
     $image,
     $start_time,
     $close_time,
     undef,
     @bids
  ) = @{read_item_file($cat, $item)};

################################
sub get_table_val

   my (
      $close_time, $image, $img_info, $title, $bids, $bid, $cat,
      $item, $row, $rowcolor, $tbl_val, $the_bid, $date, $time
   );

   
    (
       $close_time,
       $image,
       $title,
       $bids,
       $bid,
       $cat,
       $item
    ) = split /\[\]/, $_;

################################

Btw:
it's a pain to search a script for code-mistakes done by others. :'(
Keep in mind ...
you are using this script free of any charge!
So it would be nice if you could ask your questions in a more polite way.
No hard feelings - Just my very private opinion ...