See the documentation for Exporter. Basically anything you list in @EXPORT_OK will be available for import by name into other modules which use the exporting module, and anything you list in @EXPORT will be imported automatically into useing modules. It's generally considered better style to use @EXPORT_OK, or to use a named tag in %EXPORT_TAGS, but if the whole purpose of your module is to provide constants @EXPORT may be OK.
Your module might look something like this:
use base 'Exporter';
use vars qw(@EXPORT_OK %EXPORT_TAGS);
@EXPORT_OK = qw($const1);
%EXPORT_TAGS = (all => \@EXPORT_OK);