How do I use constants from a Perl module?

Question

If I define a constant in a Perl module, how do I use that constant in my main program? (Or how do I call that constant in the main program?)

Answer 1

Constants can be exported just like other package symbols. Using the standard Exporter module, you can export constants from a package like this:

package Foo; use strict; use warnings;  use base 'Exporter';  use constant CONST => 42;  our @EXPORT_OK = ('CONST');  1; 

Then, in a client script (or other module)

use Foo 'CONST'; print CONST; 

You can use the %EXPORT_TAGS hash (see the Exporter documentation) to define groups of constants that can be exported with a single import argument.

Update: Here's an example of how to use the %EXPORT_TAGS feature if you have multiple constants.

use constant LARRY => 42; use constant CURLY => 43; use constant MOE   => 44;  our @EXPORT_OK = ('LARRY', 'CURLY', 'MOE'); our %EXPORT_TAGS = ( stooges => [ 'LARRY', 'CURLY', 'MOE' ] ); 

Then you can say

use Foo ':stooges'; print "$_\n" for LARRY, CURLY, MOE; 

Answer 2

Constants are just subs with empty prototype, so they can be exported like any other sub.

# file Foo.pm package Foo; use constant BAR => 123; use Exporter qw(import); our @EXPORT_OK = qw(BAR);   # file main.pl: use Foo qw(BAR); print BAR;