Hidden features of Perl?

Answer 1

The goatse operator*:

$_ = "foo bar";
my $count =()= /[aeiou]/g; #3

or

sub foo {
    return @_;
}

$count =()= foo(qw/a b c d/); #4

It works because list assignment in scalar context yields the number of elements in the list being assigned.

* Note, not really an operator