I'm working on similar problems with Data::FormValidator. I see it's not CGI::Application::ValidateRM, but given the relationship between these two modules, maybe the following example will give you some benefit (in other words: everything I'm going to say is supposed to be useful under the hypothesis that CGI::A::VRM's interface is similar to D::FV's one):

# ... This is part of the validator profile ...
constraints => {
    ...
    holidayto => { 
        params => [ qw/ holidayfrom holidayto / ]
        constraint => sub {
            my $from = shift;
            my $to   = shift;

            # Perform some check with Date::Calc...
        }
    } ,     
    ...
}
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Note that you can supply an anonymous sub, and a list of parameters that will be passed to the subroutine itself.