Another way to prove that, what you see inside the sub through @_ is the same variables you see outside the sub:

use strict;
use warnings;

my $a;
print \$a, "\n";
somesub($a);

sub somesub {
    print \$_[0];
}
[download]

This gives:

SCALAR(0x182419c)
SCALAR(0x182419c)
[download]