You need to understand that [...] in a regex is a character class, which matches just a single character. If you write [1-31], it can match the digits 1, 2 or 3, but only one of them. See perlretut for details.

The best thing to do is to first validate the syntax, with something like /\d\d?-\d\d?\d\d?/, and then check the ranges in perl code later.