Use some code like the following (found here: http://www.merlyn.demon.co.uk/programs/what-cw.pl):

#!/usr/local/bin/perl

if (@ARGV != 3) {
    print "usage: $0 <day> <month> <year>\n";
    exit;
}

($day, $month, $year) = @ARGV;

($cw, $y) = calendarWeek($day, $month, $year);

print "CW = $cw (in year $y)\n";

exit;

#=====================================================================
+=
# subroutines
#=====================================================================
+=

#---------------------------------------------------------------------
+-
# julianDay()
#
# calculates the Julian Day, as described in the "Calendar FAQ"
# (http://www.pauahtun.org/CalendarFAQ/cal/calendar24.html, section 2.
+15.1)
#
#---------------------------------------------------------------------
+-

sub julianDay
{
    my ($day, $month, $year) = @_;

    my ($a, $y, $m);
    $a = int((14 - $month)/12);
    $y = $year + 4800 - $a;
    $m = $month + 12 * $a - 3;

    my $jd = $day + int((153 * $m + 2)/5) + 365 * $y + int($y/4)
        - int($y/100) + int($y/400) - 32045;

    return $jd;
}

#---------------------------------------------------------------------
+-
# calendarWeek()
#
# returns an array consisting of calendar-week and year, where year
# is the year in which the calendar-week actually falls
#---------------------------------------------------------------------
+-

sub calendarWeek
{
    my ($day, $month, $year) = @_;

    my $jd = julianDay($day, $month, $year);

    my($d1, $d4, $l);

    $d4 = ($jd + 31741 - ($jd % 7)) % 146097 % 36524 % 1461;
    $l = int($d4/1460);
    $d1 = (($d4 - $l) % 365) + $l;

    my $cw = int($d1/7) + 1;

    if (($month == 1) && ($cw > 51)) { $year--; }
    elsif (($month == 12) && ($cw == 1)) { $year++; }

    return ($cw, $year);
}
[download]

1. Get the module (e.g. Date::Calc) from CPAN, untar it into a temporary folder and cd into the directory
2. Issue the following commands:

   % perl Makefile.PL PREFIX=/home/USERNAME/perl-site # set your path acc
   +ordingly
   % make
   % make test
   % make install
   % make clean
   [download]

3. then, at the beginning of your program, "add" the module to your Perl distribution by adding the following line:

   use lib qw( /home/USERNAME/perl-site );
   [download]