NAME Fixed - a readonly variable that you can assign to SYNOPSIS use 5.012; use strict; use warnings; use Fixed; fix $x = 42; $x++; # croaks DESCRIPTION "Fixed" is a little like Readonly; the main difference is that you can assign to fixed variables. What?! Then how are they fixed? Because you can only assign to them once! use 5.012; use strict; use warnings; use Fixed; fix $x; # declared but not initialized given ($author) { when ("Adams") { $x = 42 } # ok when ("Heller") { $x = 22 } # ok default { $x = undef } # ok } $x = 99; # croaks, even when $x is undef Note that Fixed differentiates between a variable which has no value, and a variable explicitly set to undef. "Fixed" does not currently support arrays and hashes. (See "Internals" below for the reason.) You can of course assign an arrayref or hashref to a fixed variable, but this does not fix the contents of the array or hash. Use Readonly if you want readonly arrays and hashes. Syntax Fixed allows variables to be declared as fixed in several ways: fix $variable = $value; fix $variable; fix ($var1, $var2, ...) = ($val1, $val2, ...); fix ($var1, $var2, ...); When a single variable is declared and initialized in the same statement (i.e. the first syntax), Fixed is able to use some optimizations, so this form should be preferred when possible. Note that declaration of a variable with "fix" must be a statement on its own; "fix" cannot be slipped into the middle of an expression. if (fix $result = $search->get_result) { # no! ...; } This is a limitation inherited from Keyword::Simple. Internals The "fix" keyword is defined using Keyword::Simple and is parsed as if you'd witten: Fixed::Scalar(my $variable, $value); # ... or ... Fixed::Scalar(my $variable); If given a value, the "Fixed::Scalar" method will attempt to discover if Readonly's XS support is available, and if so will define the variable and use XS to set the scalar's "SvREADONLY" flag. If XS is not available, or no initial value is provided, "Fixed::Scalar" will fall back to Perl's "tie" mechanism. Arrays and Hashes do not have a "SvREADONLY" flag, plus the tie mechanism doesn't really have any way to differentiate between the initial list assignment to an uninitialized array or hash, and subsequent assignments. This is why "Fixed" does not support arrays or hashes. Fixed without the Syntax Hacks If you'd rather not enable the "fix" keyword and would prefer to just define fixed variables using "Fixed::Scalar(my $variable => $value)", then that's OK. Just include some empty parentheses when loading "Fixed": use Fixed (); BUGS Please report any bugs to . SEE ALSO Readonly, Readonly::XS, MooseX::SetOnce. AUTHOR Toby Inkster . COPYRIGHT AND LICENCE This software is copyright (c) 2013 by Toby Inkster. This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself. DISCLAIMER OF WARRANTIES THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.