sub foo { return }
sub bar { return undef }

my $a = foo();   # $x is undef,   if($x) is false
my @b = foo();   # @b is empty,   if(@b) is false
my $c = bar();   # $c is undef,   if($c) is false
my @d = bar();   # @d is (undef), if(@d) is true!!