The relevant sentences from the design docs (S06, introduced with commit
7846594ee4):
"Since this option [is rw] forces an argument to be required, it cannot coexist
with the ? mark to make an argument optional. (It may, however, be used with =
indicating a default, but only if the default expression represents something
that is nameable at compile time and that can bind as an lvalue, such as
CALLER::<$/> or OUTER::<$_>.)"
AFAIU the default value in question (= 60) does not qualify as a valid "is
rw"-able expression according to S06.
Rakudo makes no difference between parameters made optional with the ? mark or
those made optional by providing a default value and simply disallows 'is rw'
(introduced with commit 45c44bafbf), e.g.:
$ perl6 -e 'sub f($x is rw = OUTER::<$_>) {$x++; say $x}; $_ = 41; f'
===SORRY!=== Error while compiling -e
Cannot use 'is rw' on an optional parameter
at -e:1
