stop directly mutating $^H in strict::bits()

This improves the code in two ways:

1. Considering strict::bits is used by B::Deparse and vars.pm as
   unofficial API, the function having this side effect is arguably
   a latent bug even if it doesn’t break those callers.

2. It is harder to follow the intended logic when the function modifies
   $^H itself but also returns a value for the caller to apply to $^H.
   With this change, all branches also become entirely symmetrical.

Author: ap

lib/strict.pm

@@ -2,7 +2,7 @@ package strict;
 
 $strict::VERSION = "1.13";
 
-my ( %bitmask, %explicit_bitmask );
+my ( %bitmask, %explicit_bitmask, $explicit_bits );
 
 BEGIN {
     # Verify that we're called correctly so that strictures will work.
@@ -12,12 +12,15 @@ BEGIN {
         if __FILE__ !~ ( '(?x) \b     '.__PACKAGE__.'  \.pmc? \z' )
         && __FILE__ =~ ( '(?x) \b (?i:'.__PACKAGE__.') \.pmc? \z' );
 
+    # which strictures are actually in force
     %bitmask = (
         refs => 0x00000002,
         subs => 0x00000200,
         vars => 0x00000400,
     );
 
+    # which strictures have at some point been turned on or off explicitly
+    # and must therefore not be touched by any subsequent `use VERSION` or `no VERSION`
     %explicit_bitmask = (
         refs => 0x00000020,
         subs => 0x00000040,
@@ -38,12 +41,11 @@ BEGIN {
 }
 
 sub bits {
-    my $bits = 0;
+    my $bits = $explicit_bits = 0;
     my @wrong;
     foreach my $s (@_) {
         if (exists $bitmask{$s}) {
-            $^H |= $explicit_bitmask{$s};
-
+            $explicit_bits |= $explicit_bitmask{$s};
             $bits |= $bitmask{$s};
         }
         else {
@@ -59,14 +61,15 @@ sub bits {
 
 sub import {
     shift;
-    $^H |= @_ ? &bits : all_bits | all_explicit_bits;
+    $^H |= @_ ? &bits | $explicit_bits : all_bits | all_explicit_bits;
 }
 
 sub unimport {
     shift;
 
     if (@_) {
         $^H &= ~&bits;
+        $^H |= $explicit_bits;
     }
     else {
         $^H &= ~all_bits;