Date::Calc (or, heaven forbid, Date::Manip) is probably overkill since you only want age in years. All you really want to know is whether the birthday has passed this year.

sub age {
    # Assuming $birth_month is 0..11
    my ($birth_day, $birth_month, $birth_year) = @_;

    my ($day, $month, $year) = (localtime)[3..5];
    $year += 1900;

    my $age = $year - $birth_year;
    $age-- unless sprintf("%02d%02d", $month, $day)
               >= sprintf("%02d%02d", $birth_month, $birth_day);
    return $age;
}
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Of course, if you're only using this on a person's birthday, all you need to do is subtract their birth year from the current year :-)