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 ) ;
}