use 5.010;
use strict;
use warnings;

# 23 is an integer.
#
say digit_sum(23);

# This expression is used to test that a string consists of a single
# digit. We use it a couple of different places, so we'll get define
# it once here.
#
use constant DIGIT => qr/^[0-9]$/;

# This is the function which adds digits
#
sub digit_sum {
	my $string = shift;
	
	# Otherwise, 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);
}