+As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
+
+ my $foo = 1;
+ $ENV{'bar'} = \$foo;
+ if( ref $ENV{'bar'} ) {
+ say "Pre 5.18.0 Behaviour";
+ } else {
+ say "Post 5.18.0 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 $OLD_PERL_VERSION
+
+=item $]
+X<$]> X<$OLD_PERL_VERSION>
+
+The revision, version, and subversion of the Perl interpreter, represented
+as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
+is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
+
+This variable can be used to determine whether the Perl interpreter
+executing a script is in the right range of versions:
+
+ warn "No PerlIO!\n" if $] lt '5.008';
+
+When comparing C<$]>, string comparison operators are B<highly
+recommended>. The inherent limitations of binary floating point
+representation can sometimes lead to incorrect comparisons for some
+numbers on some architectures.
+
+See also the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
+
+See L</$^V> for a representation of the Perl version as a L<version>
+object, which allows more flexible string comparisons.
+
+The main advantage of C<$]> over C<$^V> is that it works the same on any
+version of Perl. The disadvantages are that it can't easily be compared
+to versions in other formats (e.g. literal v-strings, "v1.2.3" or
+version objects) and numeric comparisons can occasionally fail; it's good
+for string literal version checks and bad for comparing to a variable
+that hasn't been sanity-checked.
+
+The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
+reasons but its use is discouraged. (If your reason to use C<$]> is to
+run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
+be self-defeating.)
+
+Mnemonic: Is this version of perl in the right bracket?
+