# global dummy photo
my $image = $phwin->Photo(-file => '' ) or die $!;

...

################################################################################
sub next_pic {
   my $pic_file = shift @files;
   print "processing [$pic_file]\n";

#It's necessary to use the "delete" method to delete an image object 
#and free memory associated with it. Just using a lexical variable for
#storing the image object and letting the variable to go out of scope 
#or setting to undef is not sufficient.
# http://search.cpan.org/~srezic/Tk-804.033/pod/Image.pod

   $image->delete if $image->blank;

   my  $gd_image = GD::Image->new($pic_file);
   my $small_w = int($gd_image->width * 0.2);
   my $small_h = int($gd_image->height * 0.2);

   my $resized = GD::Image->new($small_w,$small_h);
      $resized->copyResized($gd_image,0,0,0,0,
              $small_w,
              $small_h,
              $gd_image->width,
              $gd_image->height);


   $image->configure(-file => undef, -data => MIME::Base64::encode($resized->jpeg()));
   
   $photo_label->configure(-image => $image );
   
   # system 'tasklist | grep perl'; # monitoring memory
}