You could do something like this:

sub cgiapp_prerun {
  my $this = shift;
  my $rm   = shift;

  eval {
    ...
  };
  if($@) {
    $this->stash->{exception} = "Error in prerun: $@";
    $this->prerun_mode("error");
  }
}

sub error {     # error-runmode
  my $this = shift;
  my $exception = shift        || 
     $this->stash->{exception} ||
     "default error message goes here";

 ... 
}
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Admittedly, this isn't that different... but it eliminates the prerun_error runmode. It also gives you the ability to call your error run mode either with an argument:

$this->error("danger! red alert!");
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or without. If no argument is given, then the error run mode looks to see if there's anything in the stash, and if so it uses it - otherwise it spits out a generic default. You could use the stash to accumulate multiple errors. For example, if you were validating a form with lots of inputs, you could do something like:

# check 'name' entry
unless ($name) {
  $this->stash->{exception} .= "bad name\n";
}

# check 'address' entry
unless ($address) {
  $this->stash->{exception} .= "bad address\n";
}

# check 'phone' entry
unless ($phone) {
  $this->stash->{exception} .= "bad phone\n";
}

return $this->error( $this->stash->{exception} ) 
  if $this->stash->{exception};

...
[download]