value in C<ENV> changes the environment for any child processes
you subsequently C<fork()> off.
+As of 5.17.3, both keys and values stored in C<%ENV> are stringified.
+
+ my $foo = 1;
+ $ENV{'bar'} = \$foo;
+ if( ref $ENV{'bar'} ) {
+ say "Pre 5.17.3 Behaviour";
+ } else {
+ say "Post 5.17.3 Behaviour";
+ }
+
+Previously, only child processes received stringified values:
+
+ my $foo = 1;
+ $ENV{'bar'} = \$foo;
+
+ # Always printed 'non ref'
+ system($^X,'-e',q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
+
+This happens because you can't really share arbitrary data structures with
+foreign processes.
+
=item $SYSTEM_FD_MAX
=item $^F