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.
In the peculiar use VERSION
form, VERSION may be either a positive decimal fraction such as 5.006, which will be compared to $]
, or a v-string of the form v5.6.1, which will be compared to $^V
(aka $PERL_VERSION). 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. Symmetrically, no VERSION
allows you to specify that you want a version of Perl older than the specified one.
Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl (that is, prior to 5.6.0) that do not support this syntax. The equivalent numeric version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current Perl version before use
ing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.)
Also, if the specified Perl version is greater than or equal to 5.9.5, use VERSION
will also load the feature
pragma and enable all features available in the requested version. See feature. Similarly, if the specified Perl version is greater than or equal to 5.11.0, strictures are enabled lexically as with use strict
(except that the strict.pm file is not actually loaded).
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. The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION
.
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. Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
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
command that unimports meanings imported by use
, i.e., it calls unimport Module 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;
Care should be taken when using the no VERSION
form of no
. It is only meant to be used to assert that the running perl is of a earlier version than its argument and not to undo the feature-enabling side effects of use VERSION
.
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.