constant::lexical - Perl pragma to declare lexical compile-time constants




  use constant::lexical DEBUG => 0;
          use constant::lexical PI => 4 * atan2 1, 1;
          use constant::lexical DEBUG => 1;

          print "Pi equals ", PI, "...\n" if DEBUG;
  print "just testing...\n" if DEBUG; # prints nothing
                                        (DEBUG is 0 again)
  use constant::lexical \%hash_of_constants;
  use constant::lexical WEEKDAYS => @weekdays; # list

  use constant::lexical { PIE        => 4 * atan2(1,1),
                          CHEESECAKE => 3 * atan2(1,1),


This module creates compile-time constants in the manner of, but makes them local to the enclosing scope.


I sometimes use these for objects that are blessed arrays, which are faster than hashes. I use constants instead of keys, but I don't want them exposed as methods, so this is where lexical constants come in handy.


This module requires perl 5.8.0 or later. If you are using a version of perl lower than 5.22.0, then you will need one of the following modules, which you can get from the CPAN:


The following three bugs have been fixed for perl 5.12.0 and higher, but are still present for older versions of perl:

  • These constants are no longer available at run time, so they won't work in a string eval (unless, of course, the use statement itself is inside the eval).

  • These constants actually are accessible to other scopes during compile-time, as in the following example:

      sub foo { print "Debugging is on\n" if &{'DEBUG'} }
              use constant::lexical DEBUG => 1;
              BEGIN { foo }
  • If you switch to another package within a constant's scope, it (the constant) will apparently disappear.

If you find any other bugs, please report them to the author via e-mail.


The idea of using objects in %^H (in the pre-5.10 code) was stolen from namespace::clean. The idea of doing cleanup in a DELETE method on a tied field hash (in the 5.10 code) was likewise stolen from namespace::clean.

Some of the code for the perl 5.12.0+ version is plagiarised from by Tom Phoenix.


Copyright (C) 2008, 2010, 2012, 2016 Father Chrysostomos (sprout at, um, cpan dot org)

This program is free software; you may redistribute or modify it (or both) under the same terms as perl.


constant, Sub::Delete, namespace::clean, Lexical::Sub

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