btrott's BEGIN solution is quite good. You also might consider making your constants.pl file a module. Simply rename it to 'constants.pm', make sure it's in the same directory as your other files, and check to see that the absolute last statement returns true (ie, if you put functions in there, add the line '1;' at the very end).

Then, change the require line to use constants;. See perlmod for more gory details.

Update: Yes, that should be PM not PL. Thanks for pointing it out, plaid and gregorovius!