Make the following modification, and your problem should be resolved. (Solution tested with Perl 5.8.0 on win32)

• Instead of defining $thumb inside sub choosethumbs, now define it outside the sub, could be at the beginning of your code:

  my $thumb;
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• Change your choosethumbs sub to this:

  sub choosethumbs {
      @selection = grep(m/.*\/$country\/.*/, @justfiles);
  
      if ($thumb) {#added four lines of code to destory the old thumb
          $thumb->destroy;
          $thumb = undef;
      } 
      $thumb = $mainframe->Thumbnail(-images => \@selection, -labels =>0
  +, -width => '75', -height => '75', -background => 'black', -command =
  +>\&openwin)->pack;
  }
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