The logic of subtracting years to determine someone age is flawed, you can't just compare the years and expect to get the right answer.

Calendars are lotsa fun, a rabbit hole down which you can lose yourself for... well, years. See the classic Calendrical Calculations by Reingold and Dershowitz.

  DB<20> use DateTime

  DB<21> $now = DateTime->now;

  DB<22> $dt = DateTime->new( year => 1964, month => 10, day => 16)

  DB<23> $duration = $now - $dt   

  DB<24> p $duration->years
55                              # not 56!
  DB<25>
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see DateTime::Duration

Here's how I'd do it...

#!/usr/bin/env perl

use strict;
use warnings;
use DateTime;
use DateTime::Format::Natural;

my $date_of_birth;
until ( defined $date_of_birth ) {
    print "When were you born?\n";
    my $parsed = DateTime::Format::Natural->new->parse_datetime( ~~<> 
+);
    printf "Did you mean %s? (Type 'y' if yes.)\n", $parsed->strftime(
+ '%a, %d %b %Y' );
    $date_of_birth = $parsed if <> =~ /y/i;
}

my $diff = DateTime->now - $date_of_birth;
printf "You are %d years old.\n", $diff->years;
[download]