I think there is an missing + in the SDL_manual, in the move handlers it should be

If you get stuck, here is working code up to the point you are at now

#!/usr/bin/perl
use strict;
use warnings;

# These will allow us to use all the modules necessary to create a GUI
+.
# They are all parts of the Perl SDL module.
# The :: are use to seperate the name of the module and its correspond
+ing member.
use SDL;
use SDL::Events;
use SDLx::App;
use SDLX::Rect;

# First is to set up the screen.
# The => operator is similiar to a comma.
# but it treates the word to the left as a quoted word.
# Even though there isn't any quotes.
my $app = SDLx::App->new(
    width        => 500,
    height       => 500,
    title        => 'PONG',
    dt           => 0.02,
    exit_on_quit => 1,
);

# The next step is to set up the paddles for each player.
# This is done in a hash reference.
# This is done so its possible to add information later.
# The numbers may need to be changed later to fit the paddles to the s
+creen.
# I am not 100% sure what -> actually does.
# Other than the fact its a deference operator.
# h is a shortened reference to height
my $player1 = {
    paddle => SDLx::Rect->new( 10, $app->h / 2, 10, 40),
    v_y => 0,
    score => 0,   
};
my $player2 = {
    paddle => SDLx::Rect->new( $app->w - 20, $app->h / 2, 10, 40),
    v_y => 0,
    score => 0,
};

# This is to create the ball
my $ball = {
    rect => SDLx::Rect->new( $app->w / 2, $app->h / 2, 10, 10 ),
    v_x => -2.7,
    v_y => 1.8,
};


# This is a show handler.
# This will render the objects to the screen.
$app->add_show_handler (
    sub {
        $app->draw_rect( [ 0, 0, $app->w, $app->h ], 0x000000FF );
        $app->draw_rect( $ball->{rect}, 0xFF0000FF );
        $app->draw_rect( $player1->{paddle}, 0xFF0000FF );
        $app->draw_rect( $player2->{paddle}, 0xFF0000FF );
        $app->update;
      }
    );
    

$app->add_move_handler( sub {
    my ( $step, $app ) = @_;
    my $paddle = $player1->{paddle};
    my $v_y = $player1->{v_y};
    $paddle->y( $paddle->y + ($v_y * $step)  );
});
    
$app->add_move_handler( sub {
    my ( $step, $app ) = @_;
    my $paddle = $player2->{paddle};
    my $v_y = $player2->{v_y};
    $paddle->y( $paddle->y + ($v_y * $step) );
});


$app->add_move_handler( sub {
  my ( $step, $app ) = @_;
  my $ball_rect = $ball->{rect};

  $ball_rect->x( $ball_rect->x + ($ball->{v_x} * $step) );
  $ball_rect->y( $ball_rect->y + ($ball->{v_y} * $step) );
 
  if ( $ball_rect->bottom >= $app->h ) {
    $ball_rect->bottom( $app->h );
    $ball->{v_y} *= -1;
  } 
  elsif ( $ball_rect->top <= 0 ) {
    $ball_rect->top( 0 );
    $ball->{v_y} *= -1;
  }
  
  elsif ( $ball_rect->right >= $app->w ) {
    $player1->{score}++;
    $ball->{v_x} *= -1;
    #reset_game();
    #return;
  }

  elsif ( $ball_rect->left <= 0 ) {
    $player2->{score}++;
    $ball->{v_x} *= -1;
    #reset_game();
    #return;
  }
});

$app->add_event_handler(
    sub {
        my ( $event, $app ) = @_;
        if ( $event->type == SDL_KEYDOWN ) {
            if ( $event->key_sym == SDLK_UP) {
               $player1->{v_y} = -2;
            }
            elsif ( $event->key_sym == SDLK_DOWN ){
               $player1->{v_y} = 2;
            }
        }
        elsif ( $event->type == SDL_KEYUP) {
          if (
            $event->key_sym == SDLK_UP
            or $event->key_sym == SDLK_DOWN
          ) {
            $player1->{v_y} = 0;
          }
       }
   }
);
$app->run;
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