Well this won't affect speed much but it removed duplicate code and increases readability.

UPDATE: my is expensive. Move it out of (important) tight loops.

my($key, $d);
while(1) {
  #Does this really need to be called each round?
  #(no dox), if so move into until
  $event->pump();
  until ($event->poll != 0) { select(undef,undef,undef, 0.01); } #micr
+osleep
  $key = $event->key_sym();
  $d      = 0;
  $d      = 1 if $event->type == SDL_KEYDOWN;

  $xplus  = $d if $key eq SDLK_RIGHT;
  $xminus = $d if $key eq SDLK_LEFT;
  $yminus = $d if $key eq SDLK_UP;
  $yplus  = $d if $key eq SDLK_DOWN;
  $fire   = $d if $key eq SDLK_SPACE;

  exit if $key eq SDLK_ESCAPE;
}
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--
perl -pew "s/\b;([mnst])/'$1/g"