I think you're missing the point of the _ prototype. In your example you don't make use of it.

The effect of _ is that if the argument is missing, $_ is passed, and thus populates @_. It does not matter if $_ is lexical. These examples may make it a bit clearer.

use 5.010;

sub old     { say "$_ " . ($_[0] // '<undef>' ) }
sub new (_) { say "$_ " . ($_[0] // '<undef>' ) }

old() for qw/ global /;
new() for qw/ global /;

local $_ = 'global';
my $_;
old() for qw/ lexical /;
new() for qw/ lexical /;

__END__
global <undef>
global global
global <undef>
global lexical
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lodin