Should I use Internals::SvREADONLY for creating readonly variables in Perl?

Question

Looking into the Const::Fast source I noticed that it used the built-in function Internals::SvREADONLY internally. Is it safe to use that function directly in my Perl script? It seems to be present in core from Perl 5.8.

my $PI = 4 * atan2 1, 1;
Internals::SvREADONLY($PI => 1);
$PI = 2.718;   # Modification of a read-only value attempted at ..

Answer 1

C:\>perldoc Internals
No documentation found for "Internals".

No.

More specifically, the package is named "Internals" for a reason. It is not intended for use outside the core. It could change without notice.

Answer 2

This is not quite answering your question, but I think it is worth mentioning so others don't experience the same pain as I have: don't use any readonly value if you're running on a version of Perl earlier than 5.10.1. Consider this little example:

{
    package Foo;
    sub foo { print "I'm in foo!\n"; }
}

use strict;
use warnings;
use Readonly;
Readonly my @classes => qw(Foo);

foreach my $class (@classes)
{
    # this dies with "Can't call method "foo" without a package or object reference"
    $class->foo;
}

Since my XS-fu is not very high, I can't explain what is going on here very coherently (but Devel::Peek shows some interesting things in the $class variable).