I suggest you look at a readily available module that would allow you to do what you want and probably even more. The module is HTML::FormValidator.

I remember using it once or twice in a few of my projects and as far as I can tell it's the right answer to these kind of problems.

You simply built form validation rules (or constraints) like this:

my $validation_rules = 
{
    customer_infos => {
        optional     =>
        [ qw( company fax country ) ],
        required     =>
        [ qw( fullname phone email address city state zipcode ) ],
        constraints  =>
        {
            email    => "email",
            fax        => "american_phone",
            phone    => "american_phone",
            zipcode    => '/^\s*\d{5}(?:[-]\d{4})?\s*$/',
            state    => "state",
        },
        defaults => {
        country => "USA",
        },
    }
};
[download]

I believe hash keys are form names (top keys) and form element names. So, the zip code match is already there for you ;-). I suggest you read the docs for better info.

"There is no system but GNU, and Linux is one of its kernels." -- Confession of Faith