Red herrings, and a wrong conclusion. It has nothing to do with either @_, or eval. It all has to do with Perl using pass by alias.

#!/usr/bin/perl

use strict;
use warnings;

my $n;

sub foo {
    $n = 0;
    return $_[0];
}

print foo(3), "\n";
$n = 3;
print foo($n), "\n";

__END__
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$_[0] is just a different name for $n, which is being modified in foo, just like your function uses eval, which sets $@.