package MyConstants;
require Exporter;
use base qw(Exporter);

@EXPORT = qw(C1 C2);
use constant C1 => 'blah';
use constant C2 => 'foo';
1;

##</code><code>##

#!/usr/bin/perl
use MyConstants.pm
require 'my_support_lib.pl';
...
# do something with the constants from MyConstants.pm
...

##</code><code>##

...
use MyConstants.pm;
...
# do something with the constants from MyConstants.pm
...