zap obsolete var, minimum Perl 5.10

Author: mohawk2

Makefile.PL

@@ -37,7 +37,7 @@ WriteMakefile(
         ? ('LICENSE'          => 'perl')
         : ()
     ),
-    MIN_PERL_VERSION          => 5.006,
+    MIN_PERL_VERSION          => 5.010,
     CONFIGURE_REQUIRES => {
         'ExtUtils::MakeMaker' => '6.64', # TEST_REQUIRES
         'Alien::Gnuplot'      => 0,