opendir(my $h, '.') or die $!;

sub a {
    my ($ref) = @_;
    $$ref = 100;
}

my $value;
a($value);
STDOUT->say($value); # => 100

a(my $value);
STDOUT->say($value); # => nothing

sub a {
    $_[0] = 100;
}

a(my $x);
print "$xn"; # => 100

sub a {
    my $x = $_[0];
    $$x = 100;
}

a(my $x);
print "$xn"; # => 100