SDL_GetKeyboardState

The whole keyboard as a string of bytes, one per scancode, each 1 when that key is down. Index it with a SDL_SCANCODE_* constant:

my $keys = SDL_GetKeyboardState();
my $left = ord substr $keys, SDL_SCANCODE_LEFT, 1;

SDL2::key_pressed($scancode) is the same question without building the string, and is what a per-frame check should use.