#!/usr/bin/perl
use strict;
use warnings;
use feature 'say';

sub calc_sunrise_sunset
{
    # some astro math here
}

my ($sunrise_hour, $sunrise_minute, $sunset_hour, $sunset_minute) = calc_sunrise_sunset();
open my $pipe,'|-','crontab','-' or die "Can't run crontab: $!";
say $pipe "0 0 * * * $0"; # keep this script running every day at midnight
say $pipe "$sunrise_hour $sunrise_minute * * * /usr/local/bin/move_curtains open"; # open at sunrise
say $pipe "$sunset_hour $sunset_minute * * * /usr/local/bin/move_curtains close"; # close at runset
close $pipe;