Const::Fast - Facility for creating read-only scalars, arrays, and hashes


version 0.014


 use Const::Fast;

 const my $foo => 'a scalar value';
 const my @bar => qw/a list value/;
 const my %buz => (a => 'hash', of => 'something');


const $var, $value

const @var, @value...

const %var, %value...

This the only function of this module and it is exported by default. It takes a scalar, array or hash lvalue as first argument, and a list of one or more values depending on the type of the first argument as the value for the variable. It will set the variable to that value and subsequently make it readonly. Arrays and hashes will be made deeply readonly.

Exporting is done using Sub::Exporter::Progressive. You may need to depend on Sub::Exporter explicitly if you need the latter's flexibility.


This module was written because I stumbled on some serious issues of Readonly that aren't easily fixable without breaking backwards compatibility in subtle ways. In particular Readonly's use of ties is a source of subtle bugs and bad performance. Instead, this module uses the builtin readonly feature of perl, making access to the variables just as fast as any normal variable without the weird side-effects of ties. Readonly can do the same for scalars when Readonly::XS is installed, but chooses not to do so in the most common case. This may change in the future if someone takes up maintenance of Readonly, and the two modules may be convergence if that happens.


Perl doesn't distinguish between restricted hashes and readonly hashes. This means that:

 use Const::Fast;
 const my %a => (foo => 1, bar => 2);
 say 1 unless $a{baz}

Will give the error "Attempt to access disallowed key 'baz' in a restricted hash". You have to use exists $a{baz} instead. This is a limitation of perl that can hopefully be solved in the future.


The interface for this module was inspired by Eric Roode's Readonly. The implementation is inspired by doing everything the opposite way Readonly does it.


Leon Timmermans <>


This software is copyright (c) 2010 by Leon Timmermans.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.