=over =item use Module VERSION LIST X X X =item use Module VERSION =item use Module LIST =item use Module 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 I be a bareword. The importation can be made conditional by using the L module. The C forces the L|/require VERSION> and L|/import LIST> to happen at compile time. The L|/require VERSION> makes sure the module is loaded into memory if it hasn't been yet. The L|/import LIST> is not a builtin; it's just an ordinary static method call into the C package to tell the module to import the list of features back into the current package. The module can implement its L|/import LIST> method any way it likes, though most modules just choose to derive their L|/import LIST> method via inheritance from the C class that is defined in the L|Exporter> module. See L. If no L|/import LIST> 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 L|/import LIST> 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 L|/use Module VERSION LIST> will call the C 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 L method|UNIVERSAL/C>, inherited from the L|UNIVERSAL> class, croaks if the given version is larger than the value of the variable C<$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 C 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 L's C method handles numeric arguments specially, performing version checks rather than treating them as things to export. Again, there is a distinction between omitting LIST (L|/import LIST> called with no arguments) and an explicit empty LIST C<()> (L|/import LIST> 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 sigtrap qw(SEGV BUS); use strict qw(subs vars refs); use subs qw(afunc blurfl); use warnings qw(all); use sort qw(stable); Some of these pseudo-modules import semantics into the current block scope (like L|strict> or L|integer>, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file). Because L|/use Module VERSION LIST> takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting a L|/use Module VERSION LIST> 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 L pragma: use if $] < 5.008, "utf8"; use if WANT_WARNINGS, warnings => qw(all); There's a corresponding L|/no MODULE VERSION LIST> declaration that unimports meanings imported by L|/use Module VERSION LIST>, i.e., it calls C<< Module->unimport(LIST) >> instead of L|/import LIST>. It behaves just as L|/import LIST> does with VERSION, an omitted or empty LIST, or no unimport method being found. no integer; no strict 'refs'; no warnings; See L for a list of standard modules and pragmas. See L for the C<-M> and C<-m> command-line options to Perl that give L|/use Module VERSION LIST> functionality from the command-line. =item use VERSION Lexically enables all features available in the requested version as defined by the L pragma, disabling any features not in the requested version's feature bundle. See L. VERSION may be either a v-string such as v5.24.1, which will be compared to L|perlvar/$^V> (aka $PERL_VERSION), or a numeric argument of the form 5.024001, which will be compared to L|perlvar/$]>. 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 L|/require VERSION>, 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 L|strict>. Similarly, if the specified Perl version is 5.35.0 or higher, L are enabled. Later use of C will override all behavior of a previous C, possibly removing the C, C, and C added by it. C does not load the F, F, or F files. In the current implementation, any explicit use of C or C overrides C, 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 C declaration be the first significant statement within a file (possibly after a C 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 5.6 This is often useful if you need to check the current Perl version before L|/use Module VERSION LIST>ing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.) Symmetrically, C 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 C form, as it is I meant to be used to assert that the running Perl is of a earlier version than its argument and I to undo the feature-enabling side effects of C. =back