Readonlyをお使いなら、s/Readonly/Const/gで同じように動くはずです。

Readonlyはないすでだみあんなモジュールですが、実装にtieを用いているため低速です。

ところが、SVには、すでにREADONLY flagが用意されているのです。これを使わぬ手はありません。

実は、Perl 5.8以降では、Internals::SvREADONLY()という関数がuseなしで使えるようになっていて、Internals::SvREADONLY($scalar, 1)で$scalarをREADONLY flagをonに、Internals::SvREADONLY($scalar, 0)でoffにできます。

ただし、これではscalarPV一つしかflagをいじれません。というわけで、同様のことをXSでやるようにするモジュールを書いたというわけです。

Enjoy!

Dan the XS Monger

NAME
    Const - Facility for creating read-only variables

VERSION
    $Id: README,v 0.1 2008/06/26 22:26:16 dankogai Exp dankogai $

SYNOPSIS
     use Const;
     Const my $sv => $initial_value;
     Const my @av => @values;
     Const my %hv => (key => value, key => value, ...);

     use Const qw/dlock dunlock/;
     # note parentheses and equal
     dlock( my $sv = $initial_value );
     dlock( my $ar = [@values] );
     dlock( my $hr = { key => value, key => value, ... } );
     dunlock $sv;
     dunlock $ar; dunlock \@av;
     dunlock $hr; dunlock \%hv;

DESCRIPTION
    Const is a drop-in replacement for Readonly. It has the same
    functionality as Readonly but istead of using "tie", it makes use of
    "SvREADONLY". Readlonly::XS does that but only to scalars while "Const"
    recursively makes all scalars in arrays and hashes.

    Note that it does not recurse on blessed references for a good reason.
    Suppose

        package Foo;
        sub new { my $pkg = shift; bless { @_ }, $pkg }
        sub get { $_[0]->{foo} }
        sub set { $_[0]->{foo} = $_[1] };

    And:

        Const my $o => Foo->new(foo=>1);

    You cannot change $o but you can still use mutators:

        $o = Foo->new(foo => 2); # BOOM!
        $o->set(2);              # OK

    If you want to make "$o->{foo}" immutable, Define Foo::new like:

        sub new {
          my $pkg = shift; 
          Const my $self = { @_ }; 
          bless $self,  $pkg;
       }

    Or consider using Moose.

EXPORT
    "Const" by default. "dlock" and "dunlock" on demand.

FUNCTIONS
  Const
    See "SYNOPSIS".

  dlock
      dlock($scalar);

    Locks $scalar and if $scalar is a reference, recursively locks
    referents.

  dunlock
    Does the opposite of "dlock".

BENCHMARK
    Unlike L <Readonly> which implements immutability via C <tie()>, Const
    just turns on read -only flag of the scalars so it is faster. Check
    t/benchmark.pl for details.

  Scalar
                Rate constant Readonly    Const  literal     glob
  constant   35461/s       --     -32%     -93%     -99%     -99%
  Readonly   52083/s      47%       --     -90%     -99%     -99%
  Const     526316/s    1384%     911%       --     -89%     -89%
  literal  5000000/s   14000%    9500%     850%       --      -0%
  glob     5000000/s   14000%    9500%     850%       0%       --
  Array w/ 1000 elements
               Rate Readonly    Const  literal     glob
      Readonly 1205/s       --     -49%     -67%     -80%
      Const    2381/s      98%       --     -36%     -60%
      literal  3704/s     207%      56%       --     -37%
      glob     5882/s     388%     147%      59%       --

  Hash w/ 1000 key-value pairs.
                Rate Readonly    Const  literal     glob
      Readonly 180/s       --     -35%     -41%     -59%
      Const    277/s      54%       --      -9%     -36%
      literal  305/s      70%      10%       --     -30%
      glob     433/s     141%      56%      42%       --

AUTHOR
    Dan Kogai, "<dankogai at dan.co.jp>"

BUGS
    Please report any bugs or feature requests to "bug-const at
    rt.cpan.org", or through the web interface at
    <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Const>. I will be
    notified, and then you'll automatically be notified of progress on your
    bug as I make changes.

SUPPORT
    You can find documentation for this module with the perldoc command.

        perldoc Const

    You can also look for information at:

    *   RT: CPAN's request tracker

        <http://rt.cpan.org/NoAuth/Bugs.html?Dist=Const>

    *   AnnoCPAN: Annotated CPAN documentation

        <http://annocpan.org/dist/Const>

    *   CPAN Ratings

        <http://cpanratings.perl.org/d/Const>

    *   Search CPAN

        <http://search.cpan.org/dist/Const>

COPYRIGHT & LICENSE
    Copyright 2008 Dan Kogai, all rights reserved.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.