## Modules and setup
use CGI;
use HTML::Template;
use strict;

$CGI::POST_MAX=1024 * 1;   # max 1k post
$CGI::DISABLE_UPLOADS = 1; # No uploads

# Your script name is already a property,
# so this isn't generally necessary -
# just call $ENV{SCRIPT_NAME}
# instead of $ThisScript;
my $ThisScript = $ENV{SCRIPT_NAME};

# better to create new CGI object this way with ->new()
my $q = CGI->new();

my $template = HTML::Template->new(
   filename => "./template.html"
);

# instead of making variables that we'll later use
# only once, let's just do the math when defining
# the template params:
$template->param(
   PreviousPicture =>
      $q->param('Picture') - 1 || NumberOfPictures(),
   PictureToView   =>
      $q->param('Picture') || 1,
   NextPicture =>
      $q->param('Picture') % NumberOfPictures() + 1,
);

print $q->header, $template->output();

{
   # we'll go ahead and cache the number for
   # future calls without re-reading the directory
   # we don't need a global variable for this, so
   # we'll limit the scope of this cached variable to
   # this block.

   my $cached_number_of_pics;

   sub NumberOfPictures {
      return $cached_number_of_pics
         if defined $cached_number_of_pics;

      opendir(DIR, "./")
         || die "Couldn't open directory: $@\n";

      my @LFiles = grep {
         not /^(\.\.?|template\.html|pictureview\.cgi)$/
      } readdir(DIR);

      closedir(DIR);

      # We don't really have a reason to continue
      # if there are no files, so die.
      die "No files -- I have no idea what to do\n"
         unless @LFiles;

      $cached_number_of_pics = @LFiles;

      # probably not necessary to force scalar
      # context, but...
      return scalar @LFiles;
   }
}