package Foo;
# ...
do_tricky_stuff or $self->_complain("Tricky stuff failed");
do_unrecoverable_stuff or return $self->_complain("Grave problem");
# ...
sub _complain {
  my $self=shift;
  my $msg=shift;
  croak($msg) if $self->fatal_warnings();
  carp($msg);
  return undef;
}