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Evaluates the BLOCK or EXPR for each element of LIST (locally
            setting $_ to each element) and returns the list value con
+sisting
...
            scalar context, returns the number of times the expression
+ was
            true.

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

exists $is_prime{$_}

and $is_prime{$_}

is_prime($_)

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