The main problem I see with this approach is that it doesn't behave the way we would expect a true lexical sub too. For example, I would expect the following to work:

sub foo { 
    my sub bar { 
        print "42\n";
    }

    return sub { bar() };
}

my $s = foo();
$s->();
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Of course that won't work with the local trick since you can't have a lexical closure over a locally modified symbol table (as far as I know.) But it would work fine if you used my $bar = sub { print "42\n" }.

I suppose a source filter to convert my sub bar { } to my $bar = sub { } would do the trick, but then you'd have to scan the rest of the block to change bar to $bar->(). Sounds like a real PITA.