FormValidator::SimpleでBETWEENやEQUAL_TOに0を使いたい
FormValidator::Simple(v0.14)でBETWEENやEQUAL_TOに0が使えない
                              
my $result = FormValidator::Simple->check( $q => [
    age => [ ['BETWEEN', 0, 3] ],
] );
                              はerrorになります
FormValidator::Simple::Validatorで
                              
    unless ( defined $start && $start =~ /^\d+$/ && defined $end && $end =~ /^\d+$/ ) {
        FormValidator::Simple::Exception->throw(
        qq/Validation BETWEEN needs two numeric arguments./
        );
    }
                              と、definedを入れてみてはどうだろう。