But do you understand the strict as the default will break a lot of old scripts too.

Yes, that falls under my earlier statement about "expectations that it could be used to run "clean" (by some definition) Perl 5 code without modification." "Clean" Perl 5 code (by almost all definitions) already uses strict, so enabling strict by default will not break it.

Backslash-escaped line endings are not currently recommended to be included in "clean" Perl 5 code. Indeed, they are not currently a part of the language at all. Thus, requiring them (even if only for some subset of mid-statement linebreaks) is something that will break existing "clean" code.

What most people here do not understand is it can be implemented completely on lexical scanner level, not affecting syntax analyser.

And what, exactly, does that have to do with anything? The change will be backwards-incompatible and break existing code. Whether it's broken by the lexical scanner or broken by the syntax analyzer is completely irrelevant. The code won't run either way.