man strict () - Perl pragma to restrict unsafe constructs

NAME

strict - Perl pragma to restrict unsafe constructs

SYNOPSIS

    use strict;


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


    use strict;
    no strict "vars";

DESCRIPTION

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. 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: 
    $bar = \&{'foo'};
    &$bar;
is allowed so that CWgoto &$AUTOLOAD would not break under stricture. This generates a compile-time error if you access a variable that wasn't declared via CWour or CWuse vars, localized via CWmy(), or wasn't 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 and local in perlfunc. 
    use strict 'vars';
    $X::foo = 1;         # ok, fully qualified
    my $foo = 10;        # ok, my() var
    local $foo = 9;      # blows up

    package Cinna;
    our $bar;                   # Declares $bar in current package
    $bar = 'HgS';               # ok, global declared via pragma
The local() generated a compile-time error because you just touched a global name without fully qualifying it. Because of their special use by sort(), the variables CW$a and CW$b are exempted from this check. This disables the poetry optimization, generating a compile-time error if you try to use a bareword identifier that's not a subroutine, unless it is a simple identifier (no colons) and that it appears in curly braces or on the left hand side of the CW=> symbol. 
    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.

HISTORY

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

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) '...'