use warnings;
use strict;
use Time::Piece qw();

for my $date (qw(2016-04-01 2016-03-25)) {
    my $t = Time::Piece->strptime($date, '%Y-%m-%d');
    my $day_of_month  = $t->day_of_month();
    my $week_of_month = int($day_of_month/7) + 1;
    print "Week $week_of_month of ", $t->fullmonth(), "\n";
}

__END__

Week 1 of April
Week 4 of March