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);
}

[download]
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re: Manipulating data from a file by tobyink
in thread Manipulating data from a file by CrimSin

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