Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to
BEGIN { require Module; Module->import( LIST ); }except that Module must be a bareword. The importation can be made conditional by using the if module.
The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin; it's just an ordinary static method call into the Module package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. If no import method can be found, then the call is skipped, even if there is an AUTOLOAD method.
If you do not want to call the package's import method (for instance, to stop your namespace from being altered), explicitly supply the empty list:
use Module ();That is exactly equivalent to
BEGIN { require Module }If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument:
use Module 12.34;is equivalent to:
BEGIN { require Module; Module->VERSION(12.34) }The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION.
The VERSION argument cannot be an arbitrary expression. It only counts as a VERSION argument if it is a version number literal, starting with either a digit or v followed by a digit. Anything that doesn't look like a version literal will be parsed as the start of the LIST. Nevertheless, many attempts to use an arbitrary expression as a VERSION argument will appear to work, because Exporter's import method handles numeric arguments specially, performing version checks rather than treating them as things to export.
Again, there is a distinction between omitting LIST (import called with no arguments) and an explicit empty LIST () (import not called). Note that there is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Some of the currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use feature qw(say signatures bitwise);
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);Some of these pseudo-modules import semantics into the current block scope (like strict or integer, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).
Because use takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting a use inside the false branch of a conditional doesn't prevent it from being processed. If a module or pragma only needs to be loaded conditionally, this can be done using the if pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);There's a corresponding no declaration that unimports meanings imported by use, i.e., it calls Module->unimport(LIST) instead of import. It behaves just as import does with VERSION, an omitted or empty LIST, or no unimport method being found.
no integer;
no strict 'refs';
no warnings;See perlmodlib for a list of standard modules and pragmas. See perlrun for the -M and -m command-line options to Perl that give use functionality from the command-line.
Lexically enables all features available in the requested version as defined by the feature pragma, disabling any features not in the requested version's feature bundle. See feature.
VERSION may be either a v-string such as v5.24.1, which will be compared to $^V (aka $PERL_VERSION), or a numeric argument of the form 5.024001, which will be compared to $]. An exception is raised if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with require, which can do a similar check at run time.
If the specified Perl version is 5.12 or higher, strictures are enabled lexically as with use strict.
If the specified Perl version is 5.35.0 or higher, warnings are enabled.
If the specified Perl version is 5.39.0 or higher, builtin functions are imported lexically as with use builtin with a corresponding version bundle.
Use of use VERSION while another is in effect is not allowed with a use v5.39; or greater version. For lower versions, use VERSION will override most behavior of a previous use VERSION, possibly removing warnings and feature effects added by it. This behavior is deprecated, and a future release of perl will disallow changing the version once one has been declared. Additionally, a use VERSION with a version less than 5.11 is not allowed after a use VERSION with a version greater than 5.11.
use VERSION does not load the feature.pm, strict.pm, warnings.pm or builtin.pm files, but instead implements the equivalent functionality directly.
In the current implementation, any explicit use of no strict overrides use VERSION, even if it comes before it. However, this may be subject to change in a future release of Perl, so new code should not rely on this fact. It is recommended that a use VERSION declaration be the first significant statement within a file (possibly after a package statement or any amount of whitespace or comment), so that its effects happen first, and other pragmata are applied after it.
Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric form was the only supported syntax, which is why you might see it in older code.
use v5.24.1; # compile time version check
use 5.24.1; # ditto
use 5.024_001; # ditto; older syntax compatible with perl 5.6This is often useful if you need to check the current Perl version before useing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.)
Symmetrically, no VERSION allows you to specify that you want a version of Perl older than the specified one. Historically this was added during early designs of the Raku language (formerly "Perl 6"), so that a Perl 5 program could begin
no 6;to declare that it is not a Perl 6 program. As the two languages have different implementations, file naming conventions, and other infrastructure, this feature is now little used in practice and should be avoided in newly-written code.
Care should be taken when using the no VERSION form, as it is only meant to be used to assert that the running Perl is of an earlier version than its argument and not to undo the feature-enabling side effects of use VERSION.