use strict;
    use strict "vars";
    use strict "refs";
    use strict "subs";
    use strict;
    no strict "vars";
    use strict 'refs';
    $ref = \$foo;
    print $$ref;        # ok
    $ref = "foo";
    print $$ref;        # runtime error; normally ok
local()
variable isn't good enough. See my and
local.
    use strict 'vars';
    $X::foo = 1;         # ok, fully qualified
    my $foo = 10;        # ok, my() var
    local $foo = 9;      # blows up
The local() generated a compile-time error because you just
touched a global name without fully qualifying it.
    use strict 'subs';
    $SIG{PIPE} = Plumber;       # blows up
    $SIG{PIPE} = "Plumber";     # just fine: bareword in curlies always ok
    $SIG{PIPE} = \&Plumber;     # preferred form