Why Dancer app fails under uWSGI + Apache?

I feel like I need to set uWSGIModifier1 5 here but did not figured out where and how?

Yes, you're right. You should set modifier1 to 5, but uwsgi docs says about mod_proxy_uwsgi:

Currently the module lacks the ability to set modifiers, though this will be fixed soon.

That means, you can't pass modifier to uWSGI instance using this method (uWSGI will use modifier 0 if not supplied)

To fix that issue, you can move to mod_uwsgi or change modifier on which psgi is loaded, using:

plugins   = 0:psgi

instead of

plugins   = psgi

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