CGI::QuickForm might be a nice option

[dazzle]

You didn't say that using another Perl module was out of the question, so how about taking a look at CGI::QuickForm? If you create the form and supply the validation criteria in the form of a subroutine, it will automatically validate and highlight the error fields for you. Here's a snippet from the CGI::QuickForm perldoc:

-VALIDATE

Optional subroutine reference. This routine is called after each individual field has been validated. It is given the fields in a name=>value hash. It should either return a simple true (valid) or false (invalid) or a two element list, the first element being a true/false value and the second value either an empty string or an (html) string which gives the reason why the record is invalid. Typically it may have this structure:

    sub valid_record {
        my %field = @_ ;
        my $valid = 1 ;
        my $why   = '' ;
        # Do some multi-field validation, e.g.
        if( $field{'colour'} eq 'blue' and
            $field{'make'} eq 'estate' ) {
          $valid = 0 ; # No blue estates available.
          $why   = '<b><i>No blue estates available</i></b>' ;
        }
        # etc.
        ( $valid, $why ) ; 
    }