package Whatever::Controller
use Mojo::Base qw(Mojolicious::Controller);

use Whatever::Thingy;
use Whatever::Exceptions;    # <- based on Class::Exception

sub do_something {
  my($this)=@_;

  my $input = $this->param("input");

  # the following constructor potentially throws an exception
  my $thingy = Whatever::Thingy->new( input => $input );

  $this->stash( thingy=> $thingy );

  $this->render(
                  format => "html",
                  template => "thingy_template",
                );
  }
}

sub render_exception {
  my($this, $ex)=@_;

  # now do something based on the class of $ex
  if($ex->isa("Whathever::Exception::SourMilk") {
  $this->render(
                  format => "html",
                  template => "exception",
                  status => 501,
                  ex => $ex,
               );
  }
  else {.....} # and so on

}

1;