refactor prereq keys into constant

Author: mohawk2

lib/ExtUtils/MakeMaker.pm

@@ -39,6 +39,8 @@ our @EXPORT_OK = qw($VERSION &neatvalue &mkbootstrap &mksymlists
                     &WriteEmptyMakefile &open_for_writing &write_file_via_tmp
                     &_sprintf562);
 
+my @PREREQ_KEYS = qw(PREREQ_PM BUILD_REQUIRES CONFIGURE_REQUIRES TEST_REQUIRES);
+
 # These will go away once the last of the Win32 & VMS specific code is
 # purged.
 my $Is_VMS     = $^O eq 'VMS';
@@ -444,7 +446,7 @@ sub new {
 
     # Cleanup all the module requirement bits
     my %key2cmr;
-    for my $key (qw(PREREQ_PM BUILD_REQUIRES CONFIGURE_REQUIRES TEST_REQUIRES)) {
+    for my $key (@PREREQ_KEYS) {
         $self->{$key}      ||= {};
         if (_has_cpan_meta_requirements) {
             my $cmr = CPAN::Meta::Requirements->from_string_hash(