use warnings;
use strict;

use RPi::Pin;
use RPi::Const qw(:all);

my $curtain_relay_pin = 5;
 
my $pin = RPi::Pin->new($curtain_relay_pin);
 
$pin->mode(OUTPUT);

if (...) {
    $pin->write(LOW);  
}
elsif ($pin->read == LOW) {
    $pin->write(HIGH);
}