perl sprintf 질문...

#!/usr/bin/perl
 
use strict;
use warnings;
 
use Time::Piece;
use Time::Seconds;
 
my $year = 2000;
my $month = 2;
 
 
sub day_iterator{
        my $day = shift;
        return sub{
                my $today = $day;
                $day += ONE_DAY;
                return $today;
        }
}
 
my $first_day = Time::Piece->strptime("$year-$month-01", "%Y-%m-%d");
my $every_day = day_iterator($first_day);
my $days_in_month = iterate_while($every_day, sub {
                $_->mon == $month and $_->year == $year });
 
sub iterate_while {
        my ($iterator, $condition) = @_;
        return sub{
                local $_;
                while (defined ($_ = $iterator->())) {
                        return ($condition->()) ? $_ : undef;
                }
        }
}
 
sub cal_week_function {
        my $next_day = shift;
        my @week;
        return sub{
                while(1){
                        my $day = $next_day->() or return splice @week;
                        @week[$day->day_of_week] = $day->day_of_month;
                        return splice @week if (@week == 7);
                }
        }
}
 
my $calendar_week = cal_week_function($days_in_month);
 
print $first_day->fullmonth, " $year\n\n";
print join '   ', Time::Piece::day_list,"\n";
 
while ( my @week = $calendar_week->()) {
        print join '   ',map{ sprintf("%3s", $_ // '')} @week, "\n";
}