#$year, $month, and $day are set to the first of any given month
my ($week,undef)=Week_of_Year($year,$month,$day);
my (undef, $tmpmonth, $firstmonday)=Monday_of_Week($week,$year);
if ($tmpmonth != $month)
{
(undef, undef, $firstmonday)=Monday_of_Week(++$week,$year);
}

#$firstmonday now contains the date of the first Monday of a given month.
#Put this in a loop or a sub or whatever and you can call it with arguments.
print "First monday of $month/$year is $firstmonday\n";