If you really need a quick and dirty version, you can just use this:

use CGI;
CGI::ReadParse();
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now all your form field inputs are in a hash called "%in".

The keys are the form field names. If you've got a field called "username", it's in $in{'username'} -- I know people will frown on this because it's not the correct way to go, not object-oriented, but hell, it's quick, isn't it?

Now all you have to do is a regular expression. if($in{'username'} !~ m/a-z0-9/) and you're done.

Watch out for multiple values from checkboxes or duplicate-name fields though.
--

($_='jjjuuusssttt annootthheer
     pppeeerrrlll haaaccckkeer')=~y/a-z//s;print;
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