Using POSIX:

$weekNum = POSIX::strftime("%V", gmtime time);
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(However, this only works on systems where the POSIX implementation meets the "Single Unix" specification. Hence my system (WinXP) just returns "%V", which is less than useful...)

Using Date::Manip:

$weekNum = UnixDate(ParseDate("today"), "%W");
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Using none of the above:

# Returns the week number of NOW
sub currentWeekNumber
{
  # Get current year, day of year (0..364/5) day of week (0..6)
  my ($year, $dayOfWeek, $dayOfYear) = (gmtime time)[5,6,7];
  # Adjust DayOfWeek from American 0==Sunday, to ISO 0==Monday
  # and year from "since 1900" to the real year
  return weekNumber(($dayOfWeek + 6) % 7, $dayOfYear, $year + 1900);
}

# Returns the week number of the specified time
# Year is the real year
# Day of week is 0..6 where 0==Monday
# Day of year is 0..364 (or 365) where 0==Jan1
sub weekNumber
{
  # Get parameters
  my ($dayOfWeek, $dayOfYear, $year) = @_;

  die if ($dayOfWeek < 0);
  die if ($dayOfWeek > 6);
  die if ($dayOfYear < 0);
  die if ($dayOfYear >= 366);
  die if ($year < 0);

  # Locate the nearest Thursday
  # (Done by locating the Monday at or before and going forwards 3 day
+s)
  my $dayOfNearestThurs = $dayOfYear - $dayOfWeek + 3;
  
  # Is nearest thursday in last year or next year?
  if ($dayOfNearestThurs < 0)
  {
    # Nearest Thurs is last year
    # We are at the start of the year
    # Adjust by the number of days in LAST year
    $dayOfNearestThurs += daysInYear($year-1);
  }
  my $daysInThisYear = daysInYear($year);
  if ($dayOfNearestThurs > $daysInThisYear)
  {
    # Nearest Thurs is next year
    # We are at the end of the year
    # Adjust by the number of days in THIS year
    $dayOfNearestThurs -= $daysInThisYear;
  }
  
  # Which week does the Thurs fall into?
  my $weekNum = int ($dayOfNearestThurs / 7);
  
  # Week numbering starts with 1
  $weekNum += 1;
   
  # Pad with 0s to force 2 digit representation
  return substr "0"x2 . $weekNum, -2;
}

# Returns the number of...
sub daysInYear
{
  return 366 unless $_[0] % 400;
  return 365 unless $_[0] % 100;
  return 366 unless $_[0] % 4;
  return 365;
}
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