sub to12h {
        local $_=shift;

        if (($_ < 0) || ($_ > 23)) {
           return ("$_ is not a valid hour");
        }

        return ($_, "AM") if $_ < 12;
        return ($_, "PM") if $_ == 12;
        return ($_ -12, "PM")
}

for (0..23) {
        print join " ", to12h($_), "\n";
}