docs: bump the minimum perl version to 5.6

It's actually been this way since at least 2012 (when a 3-argument open
was added to runtests.pl). Given the lack of complaints in the interim,
it's safe to call this 23 year old perl version the minimum.

docs/INTERNALS.md

@@ -43,7 +43,7 @@ versions of libs and build tools.
  - GNU Autoconf 2.59
  - GNU Automake 1.7
  - GNU M4       1.4
- - perl         5.004
+ - perl         5.6
  - roffit       0.5
  - nroff        any version that supports `-man [in] [out]`
  - cmake        3.7

tests/runtests.pl

@@ -74,6 +74,7 @@ BEGIN {
     }
 }
 
+use 5.006;
 use strict;
 # Promote all warnings to fatal
 use warnings FATAL => 'all';