I think I figured it out. I was loading the HTTPExceptions middleware too. That seems to catch the exception and even though it rethrows it, StackTrace then skips it.

Test code:

use strict;
use Plack::Builder;

{
  package MyApp;

  sub please_die { die "egad" }
}

my $app = sub {
  MyApp->please_die;

  [200, ['Content-type' => 'text/plain'], ['hello']]
};

builder {
  enable 'StackTrace';
  # enable 'HTTPExceptions', rethrow => 1;
  $app;
};
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