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.\"
.IX Title "strict 3perl"
.TH strict 3perl 2024-05-30 "perl v5.38.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
strict \- Perl pragma to restrict unsafe constructs
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\&    use strict;
\&
\&    use strict "vars";
\&    use strict "refs";
\&    use strict "subs";
\&
\&    use strict;
\&    no strict "vars";
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
The \f(CW\*(C`strict\*(C'\fR pragma disables certain Perl expressions that could behave
unexpectedly or are difficult to debug, turning them into errors. The
effect of this pragma is limited to the current file or scope block.
.PP
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".
.ie n .IP """strict refs""" 6
.el .IP "\f(CWstrict refs\fR" 6
.IX Item "strict refs"
This generates a runtime error if you 
use symbolic references (see perlref).
.Sp
.Vb 7
\&    use strict \*(Aqrefs\*(Aq;
\&    $ref = \e$foo;
\&    print $$ref;        # ok
\&    $ref = "foo";
\&    print $$ref;        # runtime error; normally ok
\&    $file = "STDOUT";
\&    print $file "Hi!";  # error; note: no comma after $file
.Ve
.Sp
There is one exception to this rule:
.Sp
.Vb 2
\&    $bar = \e&{\*(Aqfoo\*(Aq};
\&    &$bar;
.Ve
.Sp
is allowed so that \f(CW\*(C`goto &$AUTOLOAD\*(C'\fR would not break under stricture.
.ie n .IP """strict vars""" 6
.el .IP "\f(CWstrict vars\fR" 6
.IX Item "strict vars"
This generates a compile-time error if you access a variable that was
neither explicitly declared (using any of \f(CW\*(C`my\*(C'\fR, \f(CW\*(C`our\*(C'\fR, \f(CW\*(C`state\*(C'\fR, or \f(CW\*(C`use
vars\*(C'\fR) nor fully qualified.  (Because this is to avoid variable suicide
problems and subtle dynamic scoping issues, a merely \f(CW\*(C`local\*(C'\fR variable isn't
good enough.)  See "my" in perlfunc, "our" in perlfunc, "state" in perlfunc,
"local" in perlfunc, and vars.
.Sp
.Vb 4
\&    use strict \*(Aqvars\*(Aq;
\&    $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
\&    $bar = \*(AqHgS\*(Aq;               # ok, global declared via pragma
.Ve
.Sp
The \fBlocal()\fR generated a compile-time error because you just touched a global
name without fully qualifying it.
.Sp
Because of their special use by \fBsort()\fR, the variables \f(CW$a\fR and \f(CW$b\fR are
exempted from this check.
.ie n .IP """strict subs""" 6
.el .IP "\f(CWstrict subs\fR" 6
.IX Item "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, unless it
is a simple identifier (no colons) and that it appears in curly braces,
on the left hand side of the \f(CW\*(C`=>\*(C'\fR symbol, or has the unary minus
operator applied to it.
.Sp
.Vb 4
\&    use strict \*(Aqsubs\*(Aq;
\&    $SIG{PIPE} = Plumber;   # blows up
\&    $SIG{PIPE} = "Plumber"; # fine: quoted string is always ok
\&    $SIG{PIPE} = \e&Plumber; # preferred form
.Ve
.PP
See "Pragmatic Modules" in perlmodlib.
.SH HISTORY
.IX Header "HISTORY"
\&\f(CW\*(C`strict \*(Aqsubs\*(Aq\*(C'\fR, with Perl 5.6.1, erroneously permitted to use an unquoted
compound identifier (e.g. \f(CW\*(C`Foo::Bar\*(C'\fR) as a hash key (before \f(CW\*(C`=>\*(C'\fR or
inside curlies), but without forcing it always to a literal string.
.PP
Starting with Perl 5.8.1 strict is strict about its restrictions:
if unknown restrictions are used, the strict pragma will abort with
.PP
.Vb 1
\&    Unknown \*(Aqstrict\*(Aq tag(s) \*(Aq...\*(Aq
.Ve
.PP
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
systems.