I mean 'my' becomes redundant, isn't it?

Not really, it lets perl catch typos, for example if you have $the_variable_I_want_to_check = 0; somewhere in your code and then $the_varaible_I_want_to_check = 1; later, using strict and my will raise an error at compile time. Python would just silently allow it.

Also having automine would mean that using perl 5 code that wasn't written with strict would compile, but turn package variable into lexicals. So it would still compile but completely change and break the behaviour. Any changes introduced by perl 7 that are not backward compatible should be fatal, not silent.