JSON is not required, JSON::PP will do.

Makefile.PL

@@ -138,42 +138,52 @@ push @tests, grep !/e2e/, glob "t/*/*.t";
 
 print "done\n\n";
 
+my %checked;
+
 sub check
 {
     my ($module, $text, $version) = @_;
 
     printf "checking for %-18s %-16s .... ",
            $module, $version ? "version $version" : "";
 
-    eval "use $module";
+    {
+        local $SIG{__WARN__} = sub {};
+        eval "use $module";
+    }
     (my $mod = $module) =~ s/::/\//g;
     if (my $m = $INC{"$mod.pm"})
     {
         my $v = eval { no warnings; eval "\$${module}::VERSION" };
         printf "%-8s $m\n", $v;
         if ($version && $v < $version)
         {
-            print "\n\n\n$text\n\n";
+            print "\n\n\n$text\n" unless $checked{$text}++;
+            print "\n";
         }
     }
     else
     {
-        print "not found\n\n\n$text\n\n";
+        print "not found";
+        print "\n\n\n$text\n" unless $checked{$text}++;
+        print "\n";
     }
 };
 
-check "Storable", <<EOM;
-Storable is required to store the coverage database.  You can download
-Storable from CPAN.
+my $d = <<EOM;
+The coverage database can be stored in either Storable format or in JSON.  To
+use the Storable format, the Storable module is required.  This has been a
+core module since perl-5.8.0.  To store the database in JSON format JSON::PP
+can be used.  This has been a core module since perl-5.14.0.  If you wish, you
+may install the JSON module which will be used in preference.  This may be
+configured to make database operations faster.  If both formats are available,
+JSON will be preferred by default.  The JSON format should be portable between
+all systems, whereas Storable format may not be.
 EOM
 
-check "JSON", <<EOM;
-JSON is required to store the coverage database in a portable format.
-This is necessary in order to merge coverage data from incompatible systems.
-Storing the database in a portable format will not be available until you
-install JSON.  In the meantime you may continue to use the rest of
-Devel::Cover.
-EOM
+check "Storable", $d;
+check "JSON",     $d;
+check "JSON::PP", $d;
 
 check "Digest::MD5", <<EOM;
 Digest::MD5 is required to check whether covered files have changed.  You can
@@ -207,8 +217,7 @@ PPI::HTML or Perl::Tidy, available from the CPAN.  In the meantime you
 may continue to use the rest of Devel::Cover.
 EOM
 
-check "PPI::HTML", $m, "1.07";
-
+check "PPI::HTML",  $m, "1.07";
 check "Perl::Tidy", $m, "20060719";
 
 print "checking for Pod::Coverage      version 0.06     .... ";

dist.ini

@@ -48,7 +48,6 @@ Pod::Coverage                 = 0.06
 Pod::Coverage::CountParents   = 0
 Parallel::Iterator            = 0
 JSON::PP                      = 0
-JSON::XS                      = 0
 Test::Differences             = 0           ; make sure it gets added
 [Prereqs / ConfigureRequires]
 ExtUtils::MakeMaker           = 0

lib/Devel/Cover/DB/IO.pm

@@ -16,10 +16,9 @@ my $Format;
 
 BEGIN
 {
-    $Format = "JSON"     if eval "use JSON; 1";
-    # warn "JSON available\n" if $INC{"JSON.pm"};
+    $Format = "JSON"     if              eval "use JSON; 1";
+    $Format = "JSON"     if !$Format and eval "use JSON:PP; 1";
     $Format = "Storable" if !$Format and eval "use Storable; 1";
-    # warn "Storable available\n" if $INC{"Storable.pm"};
     die "Can't load either JSON or Storable" unless $Format;
 }
 

lib/Devel/Cover/DB/IO/JSON.pm

@@ -11,10 +11,18 @@ use strict;
 use warnings;
 
 use Fcntl ":flock";
-use JSON;
 
 # VERSION
 
+my $Format;
+
+BEGIN
+{
+    $Format = "JSON"     if              eval "use JSON; 1";
+    $Format = "JSON::PP" if !$Format and eval "use JSON::PP; 1";
+    die "Can't load either JSON or JSON::PP" unless $Format;
+}
+
 sub new
 {
     my $class = shift;
@@ -30,7 +38,9 @@ sub read
     open my $fh, "<", $file or die "Can't open $file: $!";
     flock($fh, LOCK_SH) or die "Cannot lock file: $!\n";
     local $/;
-    my $data = JSON::decode_json(<$fh>);
+    my $data = $Format eq "JSON"
+        ? JSON::decode_json(<$fh>)
+        : JSON::PP::decode_json(<$fh>);
     close $fh or die "Can't close $file: $!";
     $data
 }
@@ -40,7 +50,8 @@ sub write
     my $self = shift;
     my ($data, $file) = @_;
 
-    my $json = JSON->new->utf8->allow_blessed;
+    my $json = $Format eq "JSON" ? JSON->new : JSON::PP->new;
+    $json->utf8->allow_blessed;
     $json->ascii->pretty->canonical if $self->{options} =~ /\bpretty\b/i;
     open my $fh, ">", $file or die "Can't open $file: $!";
     flock($fh, LOCK_EX) or die "Cannot lock file: $!\n";