My Bad. Your $slides are just pointers to ppt slides. Try

 my @slides;
    my $j = 1;
    my $slide;
    
    while ($addpres->Slides($j)){$slides[$j] = $addpres->Slides($j);$j
+++};  

    my $k = 1;
    foreach $slide (@slides){ 
      # make a new slide
      my $s = $pres -> Slides->Add($slideIndex,$slides[$k]->Layout) 
      $slides[$k]->Copy; #get what you want
      $s->Paste;         #save it
       $slideIndex++; $k++;
     }
[download]

I always forget the details of the PPT DOM details.