sub foo { "foo" }
$fooref = \*foo;
&$fooref();

##</code><code>##

Not a CODE reference at testing.pl line 3.

##</code><code>##

$bar = 1;
$barref = \*bar;
*$barref = sub { "bar" };
&$barref();

##</code><code>##

Not a CODE reference at testing.pl line 4.