my $days = 71;
my $firstday = 5;
my @daycounts = (0) x 7; # initialise 7 array elements to 0

for my $day_num( $firstday .. ($days+$firstday-1) ) {
    $daycounts[$day_num%7]++;
}

# now print it out nice and pretty

my %weekdays = (
    1 => Monday,
    2 => Tuesday,
    3 => Wednesday,
    4 => Thursday,
    5 => Friday,
    6 => Saturday,
    0 => Sunday);

for my $day_num(0..6) {
    print "$daycounts[$day_num]\t$weekdays{$day_num}\n";
}

# here it is as a one liner
$daycounts[$_%7]++ for $firstday..($days+$firstday-1);