use Date::Calc qw/:all/;

.
.

my @date = (Localtime)[0..2];

.
.

#   iterate through system accounts from the database

while (my $account = $accounts->fetchrow_hashref) {
    my $plan = $plans[ $account->{'planid'} ];

    #   calculate the delta day count between the current date and the 
    #   date of account activation

    my $delta = Delta_Days((split '-', $account->{'activationdate'}), @date);
    if ($delta == 0) {

        #   the delta day count will equal zero on the same day as the 
        #   account activation - this is useful for when you also want to 
        #   incorporate the sending of a welcome email or alike

    } else {

        #   within this system each billing plans has a period value which
        #   represents the number of billing systems over a single annual 
        #   period - if you are wanting to send out these emails on a 
        #   monthly basis, this variable can be replaced with the number 
        #   twelve

        if ($plan->{'fee'} && $plan->{'period'}) {

            #   here is the actual calculation itself, the modulus result 
            #   of which will equal zero on days when an email should be 
            #   sent to the user

            my $calc = $delta % int (Days_in_Year((@date)[0..1]) / $plan->{'period'});
            if ($calc == 0) {

                #   send the email to the owner of this account
            }
        }   
    }
}