Bug 8063 - Followup - Make C4/Letters.pm conform to PBP

Bug 8063 - Followup - Make C4/Letters.pm conform to PBP

Perlcritic reported the following errors:
Subroutine prototypes used at line 96, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 120, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 152, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 173, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 190, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 227, column 1.  See page 194 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 228, column 31.  See page 199 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 229, column 31.  See page 199 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 236, column 9.  See page 199 of PBP.  (Severity: 5)
Subroutine prototypes used at line 668, column 1.  See page 194 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 669, column 27.  See page 199 of PBP.  (Severity: 5)
Subroutine prototypes used at line 719, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 865, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 899, column 1.  See page 194 of PBP.  (Severity: 5)
Subroutine prototypes used at line 981, column 1.  See page 194 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 982, column 28.  See page 199 of PBP.  (Severity: 5)
Subroutine prototypes used at line 1000, column 1.  See page 194 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 1001, column 27.  See page 199 of PBP.  (Severity: 5)
"return" statement with explicit "undef" at line 1004, column 9.  See page 199 of PBP.  (Severity: 5)

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>