Thanks all for your answers! i understand now better the constant module.

Joost: i began to use a variable for holding this MONTH informations, but i feel it would be more "pretty" (if this has a sens :o)) and understable to use a constant, so i change for 'use constant ...'.

i cannot use Hash::Util as i develop extensions for a Rational product (clearquest), that's embedded with a perl 5.6.1.