There is no reason to roll this code yourself. None. Use one of the many, many excellent date/time modules on CPAN. For example, if you were to use Date::Calc for this, it would go something like:

use Date::Calc qw(
    Days_In_Month
);

if ($days > Days_In_Month($year, $month))
{
    # Bitch and moan
}
[download]

You will need to supply the year, because February changes how many days it has, depending on the year. (It's fickle like that.)