Sane default are important for beginners, and for people who don't exactly want to become experts in a language. This is a LOT of people (for example, various biologists that frequently post on this forum).

Besides the default, at least for strict and warnings, does make sense. In one-liners and in scripts that have five lines

Does such a big and complex language as Perl have a future in 'one-liners and scripts that have five lines'? Why does it have to be optimized for 'one-liners'? What makes it better than Bash in that role? Besides, one-liners work just fine even if you write them as if you had strictures and warnings enabled.

are just an unimportant and ignorable part of the boilerplate

'use strict; use warnings' is probably the most frequent advice ever in the history of Perl. These features are especially good for people who don't yet have good knowledge of Perl. And it is exactly these people that are the future. On the other hand, having to type 'my' in one-liners is no big deal at all.