Perl 5.001

[p5sagit/p5-mst-13.2.git] / pod / modpods / strict.pod

=head1 NAME

strict - Perl pragma to restrict unsafe constructs

=head1 SYNOPSIS

    use strict;

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

    use strict;
    no strict "vars";

=head1 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".

=over 6

=item C<strict refs>

This generates a runtime error if you 
use symbolic references (see L<perlref>).

    use strict 'refs';
    $ref = \$foo;
    print $$ref;	# ok
    $ref = "foo";
    print $$ref;	# runtime error; normally ok

=item C<strict vars>

This generates a compile-time error if you access a variable that wasn't
localized via C<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 L<perlfunc/my> and
L<perlfunc/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.

=item C<strict subs>

This disables the poetry optimization,
generating a compile-time error if you 
try to use a bareword identifier that's not a subroutine.

    use strict 'subs';
    $SIG{PIPE} = Plumber;   	# blows up
    $SIG{"PIPE"} = "Plumber"; 	# just fine

=back

See L<perlmod/Pragmatic Modules>.