Strangely, I'm replying to myself:

The answer to my question is yes, one must write a custom constraint. I got this reply from the module author (Mark Stosberg):

Fred, if using the 'dependencies' system doesn't solve your problem, I think you are right. You could write a custom constraint for b or c, such that they only kick-in if A is present.

He also suggested that I propose any new functionality to the maintainers list.

Solving my problem I used the following code (see Data::FormValidator for explaination)

# .. 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  
}
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