This Dancer2 plugin provides a method for deferring a one-time message across a
redirect. It is similar to "flash" messages, but without the race conditions
that can result from multiple tabs in a browser or from AJAX requests. It is
similar in design to Catalyst::Plugin::StatusMessage, but adapted for Dancer2.

It works by creating a unique message ID within the session that holds deferred
data. The message ID is automatically added as a query parameter to redirection
requests. It's sort of like a session within a session, but tied to a request
rather than global to the browser. (It will even chain across multiple
redirects.)