[perl] Require at least Perl 5.8.0

As discussed and agreed in #1629, it's become hard to test with Perl
5.6 or earlier, such old versions are no longer in active use, and
4.1.0 is an appropriate time to make such a change.

I've dropped the compatibility code that was obvious to me, but there's
probably more that can be cleaned up now.

Examples/test-suite/perl5/packageoption_runme.pl

@@ -5,15 +5,6 @@ use Test::More tests => 4;
 BEGIN { use_ok('packageoption_a'); }
 BEGIN { use_ok('packageoption_b'); }
 
-# Workaround for 
-#   ok( not (expression) , "test description" );
-# does not working in older versions of Perl, eg 5.004_04
-sub ok_not ($;$) {
-    my($test, $name) = @_;
-    $test = not $test;
-    ok($test, $name);
-}
-
 my $a = CommonPackage::A->new();
 
 isa_ok($a, 'CommonPackage::A');