Yes, it seems to work as advertised:

17:39 >perl -Mstrict -wE "my $s = 42; say $s; ++$s; say $s;"
42
43

17:39 >perl -Mstrict -MConst::Fast -wE "const my $s => 42; say $s; ++$
+s; say $s;"
42
Modification of a read-only value attempted at -e line 1.

17:39 >
[download]

How does that help?

Const::Fast is basically just a wrapper around Internals::SvREADONLY.

$ perldoc -m Const::Fast | grep SvREADONLY
    if (my $reftype = reftype $_[0] and not blessed($_[0]) and not &In
+ternals::SvREADONLY($_[0])) {
        &Internals::SvREADONLY($_[0], 1);
    Internals::SvREADONLY($_[0], 1);
    croak 'Attempt to reassign a readonly variable' if &Internals::SvR
+EADONLY($_[0]);
[download]

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

Yes, Internals::SvREADONLY(..., 1) works to correctly make a writeable variable read-only:

Read more... (783 Bytes)

And Internals::SvREADONLY(..., 0) then works to make the variable writeable again:

Read more... (756 Bytes)

But it doesn’t work on a variable which is aliased to a literal:

Read more... (741 Bytes)

So Internals::SvREADONLY() “works” under Windows; but (it would appear) its effect on an aliased variable is not portable?

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,