New paddle position also depends on $step which is the time slice.

When you press UP-arrow this checks only the 1st condition. When you press DOWN-arrow it checks both conditions. Could that be affecting $step? Ideally no but try print $step and see.

            if($event-> key_sym == SDLK_UP){
                $player1->{v_y} = -2;
            }        
            elsif($event->key_sym == SDLK_DOWN){                      
+                      
                $player1->{v_y} = 2;
            }
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bw, bliako