readonly

readonly($var) - returns true if $var is read-only.

The true value is not 1. Exactly as in perl5, it is the raw internal flag word, so a read-only scalar yields a number such as 134283264. Test it for truth; never compare it against 1. Takes exactly one argument.

sub foo { readonly($_[0]) }
$ro = foo($bar);                    # false
$ro = foo(0);                       # true