my @months = qw(JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC);

sub change_num_to_month {
     my $num_month = shift;
     return $months[$num_month - 1];
}

sub add_month {
    my $month = shift;
    my $month_num = change_month_to_num($month);
    $month_num++;
    $month_num = 1 if $month_num > 12;
    return change_num_to_month($month_num);
}

# And very similarly for subtract month