It MUST be unnecessary to declare all lexical variables as lexical if 'use strict' is ON by default.
You've already seen several (very good) arguments against your proposal in previous responses, so I won't reiterate them here, but what is your argument for undermining strict with automine? Is there any reason other than "some subset of (mostly older and unmaintained, I'd wager) existing Perl code won't run under strict"?
Also, I can't help noticing that even the post you linked to as an explanation of automine calls it a bad idea. ("I think that, in Perl, an automine pragma would be very confusing... Whatever the merits of defaulting variables to local scope might be, doing so in Perl would likely be too confusing.")