# called just after an eval
sub catch {
  # don't catch if no error
  return 0 unless defined $@;
  
  # checks if $@->isa(one of the type in @_);
  UNIVERSAL::isa($@,$_) && return 1 for @_;
  
  # otherwise re-throws
  die $@;
}

# later on ...
while (my ($name,$val) = each %input) {
  if(eval { validate($val) }) { 
    $session->set($name, $val);

  } elsif(catch('ARRAY','E')) { 
    $errors{$name} = $@;        
    $action = 'reshow';
  } 
} 

# ...
eval { $nav->$action() } 
  or catch('E::Nav') and do { $error = $@; $nav->reshow };

# ... other things I haven't thought of ...