use Data::Dumper;

my @a;
@a = (undef) x 5;
my @b;
$#b = 4;

sub modify {
    $_ = 5 for @_;
}

modify(@a);
print Dumper \@a;
modify(@b);
print Dumper \@b;

##</code><code>##

$VAR1 = [
          5,
          5,
          5,
          5,
          5
        ];
Modification of a read-only value attempted at script.pl line 12.