# .. snip
# Part of my hash to pass to D::FV

   constraints => {
      dn_id    => 'OK_int',
      b        => { 
        constraint  => 'custom_constraint_sub_name',
        params      => [ qw/ b a / ],
                  },
      c        => { 
        constraint  => 'custom_constraint_sub_name',
        params      => [ qw/ c a / ],
                  },
   }

# .. snip ...

# Defining my constraint routine elsewhere:

#################################
sub match_custom_constraint_sub_name {
  my $val = shift;
  my $a = shift;
  if ( $a eq 'N' ) {
        return  match_another_custom_constraint($val) ;
  } else {
        return "0";  # use this default value  
}