While using CGI.pm is a good thing, you still made it more complicated than it should be, at least if you ask me ...

use strict;
use CGI;

my $q = new CGI;
my $form = $q->Vars();

if(exists $form->{parameter_name}){
   #do whatever
}
[download]

Of course, you can also use Data::FormValidator. It's suggested if you need more complicated form validation.