Bug 6679: Fix 11 perlcritic violations in C4/Dates.pm

Bug 6679: Fix 11 perlcritic violations in C4/Dates.pm

- Subroutine prototypes used at line 82, column 1.  See page 194 of PBP.  (Severity: 5)

- "return" statement with explicit "undef" at line 102, column 28.  See page 199 of PBP.  (Severity: 5)

- Expression form of "eval" at line 108, column 20.  See page 161 of PBP.  (Severity: 5)

- Variable declared in conditional statement at line 126, column 5.  Declare variables outside of the condition.  (Severity: 5)

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>