doc: Bump minimal perl version to use to 5.10.0

The current version in Debian oldstable (lenny) is already 5.10.0, once
the current Debian stable release becomes oldstable (squeeze) it will
be 5.10.1, so bumping it is pretty safe and conservative.

doc/coding-style.txt

@@ -206,7 +206,7 @@ see what operation is being done:
 		foo();
 
 
-Dpkg Perl coding style 2010-05-10
+Dpkg Perl coding style 2013-01-04
 ======================
 
 General
@@ -243,7 +243,7 @@ Perl version
 
 We don't want to impose a too-recent Perl version, so only use features
 supported by the Perl version that is currently in Debian oldstable when
-possible. Currently that means Perl 5.8.8.
+possible. Currently that means Perl 5.10.0.
 
 Object methods
 ~~~~~~~~~~~~~~