use strict;

           use strict "vars";
           use strict "refs";
           use strict "subs";

           use strict;
           no strict "vars";

       If no import list is supplied, all possible restrictions are assumed.
       (This is the safest mode to operate in, but is sometimes too strict for
       casual programming.)  Currently, there are three possible things to be
       strict about:  "subs", "vars", and "refs".

       "strict refs"
             This generates a runtime error if you use symbolic references
             (see perlref).

                 use strict 'refs';
                 $ref = \$foo;
                 print $$ref;        # ok
                 $ref = "foo";
                 print $$ref;        # runtime error; normally ok
                 $file = "STDOUT";
                 print $file "Hi!";  # error; note: no comma after $file

             There is one exception to this rule:

                 $bar = \&{'foo'};

             is allowed so that "goto &$AUTOLOAD" would not break under

       "strict vars"
             This generates a compile-time error if you access a variable that
             was neither explicitly declared (using any of "my", "our",
             "state", or "use vars") nor fully qualified.  (Because this is to
             avoid variable suicide problems and subtle dynamic scoping
             issues, a merely "local" variable isn't good enough.)  See "my"
             in perlfunc, "our" in perlfunc, "state" in perlfunc, "local" in
             perlfunc, and vars.

                 use strict 'vars';
                 $X::foo = 1;         # ok, fully qualified
                 my $foo = 10;        # ok, my() var
                 local $baz = 9;      # blows up, $baz not declared before

                 package Cinna;
                 our $bar;                   # Declares $bar in current package
                 $bar = 'HgS';               # ok, global declared via pragma


                 use strict 'subs';
                 $SIG{PIPE} = Plumber;       # blows up
                 $SIG{PIPE} = "Plumber";     # just fine: quoted string is always ok
                 $SIG{PIPE} = \&Plumber;     # preferred form

       See "Pragmatic Modules" in perlmodlib.

       "strict 'subs'", with Perl 5.6.1, erroneously permitted to use an
       unquoted compound identifier (e.g. "Foo::Bar") as a hash key (before
       "=>" or inside curlies), but without forcing it always to a literal

       Starting with Perl 5.8.1 strict is strict about its restrictions: if
       unknown restrictions are used, the strict pragma will abort with

           Unknown 'strict' tag(s) '...'

       As of version 1.04 (Perl 5.10), strict verifies that it is used as
       "strict" to avoid the dreaded Strict trap on case insensitive file

perl v5.18.2                      2014-01-06                     strict(3perl)
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