strict(3)

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 the perlref manpage).

    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 "goto &$AUTOLOAD" would not break under stricture. This generates a compile-time error if you access a variable that wasn't declared via "our" or "use vars", localized via "my()", 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 the my entry in the perlfunc manpage and the local entry in the perlfunc manpage.

    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 $a and $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 appears in curly braces or on the left hand side of the "=>" symbol.

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

See the Pragmatic Modules entry in the perlmodlib manpage.