=over

=item use Module VERSION LIST
X<use> X<module> X<import>

=item use Module VERSION

=item use Module LIST

=item use Module

=item use VERSION

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<must> be a bareword.
The importation can be made conditional; see L<if>.

In the peculiar C<use VERSION> form, VERSION may be either a positive
decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
of the form v5.6.1, which will be compared to C<$^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 L</require>, which can do a similar check at run time.
Symmetrically, C<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
C<use>ing library modules that won't work with older versions of Perl.
(We try not to do this more than we have to.)

C<use VERSION> also enables all features available in the requested
version as defined by the C<feature> pragma, disabling any features
not in the requested version's feature bundle.  See L<feature>.
Similarly, if the specified Perl version is greater than or equal to
5.11.0, strictures are enabled lexically as
with C<use strict>.  Any explicit use of
C<use strict> or C<no strict> overrides C<use VERSION>, even if it comes
before it.  In both cases, the F<feature.pm> and F<strict.pm> files are
not actually loaded.

The C<BEGIN> forces the C<require> and C<import> to happen at compile time.  The
C<require> makes sure the module is loaded into memory if it hasn't been
yet.  The C<import> is not a builtin; it's just an ordinary static method
call into the C<Module> package to tell the module to import the list of
features back into the current package.  The module can implement its
C<import> method any way it likes, though most modules just choose to
derive their C<import> method via inheritance from the C<Exporter> class that
is defined in the C<Exporter> module.  See L<Exporter>.  If no C<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 C<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
C<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 C<$Module::VERSION>.

Again, there is a distinction between omitting LIST (C<import> called
with no arguments) and an explicit empty LIST C<()> (C<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 C<strict> or C<integer>, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).

Because C<use> takes effect at compile time, it doesn't respect the
ordinary flow control of the code being compiled.  In particular, putting
a C<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 L<if> pragma:

    use if $] < 5.008, "utf8";
    use if WANT_WARNINGS, warnings => qw(all);

There's a corresponding C<no> declaration that unimports meanings imported
by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
It behaves just as C<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 C<no VERSION> form of C<no>.  It is
I<only> meant to be used to assert that the running Perl is of a earlier
version than its argument and I<not> to undo the feature-enabling side effects
of C<use VERSION>.

See L<perlmodlib> for a list of standard modules and pragmas.  See L<perlrun>
for the C<-M> and C<-m> command-line options to Perl that give C<use>
functionality from the command-line.

=back