The maths part isn't especially tricky...
use 5.010;
use strict;
use warnings;
# This expression is used to test if a string consists of a single
# digit. We use it a couple of different places, so we'll just define
# it once as a constant.
#
use constant DIGIT => qr/^[0-9]$/;
# Let's assume you already have the date in an all numeric format.
#
say digit_sum('01-06-1980');
# This is the function which adds digits
#
sub digit_sum {
my $string = shift;
# First, split into digits.
my @digits =
grep { $_ =~ DIGIT } # keep only the numeric characters
split '', $string; # split into characters
# Add the digits together.
my $sum = 0;
$sum += $_ for @digits;
# Handle the trivial case.
# If $sum is just a single digit, return it as-is.
return $sum if $sum =~ DIGIT;
# Otherwise, recurse.
return digit_sum($sum);
}
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'