anyway, hint from ctilmes is spot on
https://metacpan.org/pod/Mojolicious::Guides::Growing#Simplified-application-class
https://metacpan.org/pod/Mojolicious::Routes::Route#under
https://metacpan.org/pod/Mojolicious::Guides::Tutorial#Under
https://metacpan.org/pod/Mojolicious::Guides::Routing#Under
https://metacpan.org/pod/Mojolicious::Routes::Route#name

I consider Routing to a callback a bug (in Mojolicious 6.14) since it could do something useful like mention a file and line number ; Maybe the latest version of Mojolicious 6.33 does this

Nope, it still only says "Routing to a callback" in Mojolicious 6.33 , even if you give it a name, thats two bugs :D

So here it is , self contained , based on doc example

This is how it runs, first the working version from docs, then the breaky version

You see, you can't just return true or false from under, you have to render something or redirect somewhere

Yes, particularly I missed https://metacpan.org/pod/Mojolicious::Guides::Growing#Simplified-application-class
All is there.
Thank you very much.
PC

Thank you very much! I study that.
PC

I thank you very much. You show how to reproduce my mistake under self contained minimal mojolicious example.
But I know how to do under Mojolicious::Lite (just follow the example of the doc).
My problem seems to me my failure to expand the exmple to "full" Mojolicious.
...But maybe, I am wrong myself about my mistake!