my $image = Image::Magick->new;

foreach my $pic (@pics){
   
   my $ok;
   $ok = $image->Read($pic) and warn ($ok);
   $image->Scale(geometry => '100x100');
   $ok = $image->Write($thumb) and warn ($ok);
   
   # this is the key line
   undef @$image; #needed if $image is created outside loop