You should call keypad(1) so you can compare returned key code to exported constants.

use strict;
use Curses;

my $screen = Curses->new();

# Enable keypad functionality
keypad(1);

while (my $key = getch()) {
        do { printw("KEY_LEFT\n");  refresh(); next } if $key eq KEY_L
+EFT;
        do { printw("KEY_RIGHT\n"); refresh(); next } if $key eq KEY_R
+IGHT;
        last;
}

exit 0;
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BR