Don't count on a specific environment variable existing in C<%ENV>.
Don't count on C<%ENV> entries being case-sensitive, or even
case-preserving. Don't try to clear %ENV by saying C<%ENV = ();>, or,
-if you really have to, make it conditional on C<$^O ne 'vms'> since in
+if you really have to, make it conditional on C<$^O ne 'VMS'> since in
VMS the C<%ENV> table is much more than a per-process key-value string
table.
(You can't just say C<$ENV{$key} = $ENV{$key}>, since the
Perl optimizer is smart enough to elide the expression.)
-Don't try to clear C<%ENV> by saying C<%ENV = ();> Bad things will
-happen because parts of C<%ENV> are per-process, parts of it are
-per-group, and parts of it are system-wide. Without some serious
-rights, it won't work, or with the rights, it does, but the system
-will effectively die.
+Don't try to clear C<%ENV> by saying C<%ENV = ();>, it will throw
+a fatal error. This is equivalent to doing the following from DCL:
+
+ DELETE/LOGICAL *
+
+You can imagine how bad things would be if, for example, the SYS$MANAGER
+or SYS$SYSTEM logicals were deleted.
At present, the first time you iterate over %ENV using
C<keys>, or C<values>, you will incur a time penalty as all