Actually constant.pm has very little overhead. It just declares your constants as functions (within the correct namespace even) which return their constant value. Perl is slick enough to inline these, so your only overhead is pulling in the extra module. You can eliminate this by doing it by hand, if you really feel the need.
The simple way then is too just declare the funcs your self, thusly:sub C() { 299792458 } # Meters/Second