Evaluates the BLOCK or EXPR for each element of LIST (locally
            setting $_ to each element) and returns the list value consisting
            of those elements for which the expression evaluated to true. In
            scalar context, returns the number of times the expression was
            true.


##</code><code>##

{
  ( exists $is_prime{$_} and $is_prime{$_} )
  or 
  ( $is_prime{$_} = is_prime($_) )
}

##</code><code>##

exists $is_prime{$_}

##</code><code>##

and $is_prime{$_}

##</code><code>##

is_prime($_)

##</code><code>##

$is_prime{$_} =
# the assigned value is the result of is_prime($_) if that function returned true