This is actually very cool:

package Constants;

use base 'Exporter';
our @EXPORT = grep {
    substr( $_, 0, 2 ) eq 'C_'
 && __PACKAGE__->can( $_ );
} keys %Constants::;
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The way I used to do it was to essentially use a source filter: I'd have a BEGIN block inside my Definitions.pm file read the file itself in and look for "use constant" lines:
Constant Amusement (auto export trick)

Anyone doing a lot of work with perl constants might want to skim through those talk notes, by the way. And in general, these days I'd recommend staying away from constants altogether -- their utility rarely makes up for their annoyances (e.g no sigil, so they don't interpolate very smoothly). Also a case can be made for keeping definitions of things in a YAML file instead of using perl code to store configuration (programmatic modification of YAML is a lot easier than parsing perl).