#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::Piece;

my $month = 10;
my $year = 2013;

for (1 .. 7) { # Time::Piece->wday returns: Sunday = 1, ..., Saturday = 7
    print +($_ - Time::Piece->strptime("$year$month" . '01', '%Y%m%d')->wday) % 7 + 1;
}

##</code><code>##

6
7
1
2
3
4
5

##</code><code>##

$ cal 10 2013
    October 2013
Su Mo Tu We Th Fr Sa
       1  2  3  4  5
 6  7  8  9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31