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Reconciling the Cwd/File::Spec differences
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1=head1 NAME
2
f102b883 3perlmod - Perl modules (packages and symbol tables)
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4
5=head1 DESCRIPTION
6
7=head2 Packages
8
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9Perl provides a mechanism for alternative namespaces to protect
10packages from stomping on each other's variables. In fact, there's
bc8df162 11really no such thing as a global variable in Perl. The package
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12statement declares the compilation unit as being in the given
13namespace. The scope of the package declaration is from the
14declaration itself through the end of the enclosing block, C<eval>,
15or file, whichever comes first (the same scope as the my() and
16local() operators). Unqualified dynamic identifiers will be in
17this namespace, except for those few identifiers that if unqualified,
18default to the main package instead of the current one as described
19below. A package statement affects only dynamic variables--including
20those you've used local() on--but I<not> lexical variables created
21with my(). Typically it would be the first declaration in a file
22included by the C<do>, C<require>, or C<use> operators. You can
23switch into a package in more than one place; it merely influences
24which symbol table is used by the compiler for the rest of that
25block. You can refer to variables and filehandles in other packages
26by prefixing the identifier with the package name and a double
27colon: C<$Package::Variable>. If the package name is null, the
28C<main> package is assumed. That is, C<$::sail> is equivalent to
29C<$main::sail>.
a0d0e21e 30
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31The old package delimiter was a single quote, but double colon is now the
32preferred delimiter, in part because it's more readable to humans, and
33in part because it's more readable to B<emacs> macros. It also makes C++
34programmers feel like they know what's going on--as opposed to using the
35single quote as separator, which was there to make Ada programmers feel
36like they knew what's going on. Because the old-fashioned syntax is still
37supported for backwards compatibility, if you try to use a string like
38C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
39the $s variable in package C<owner>, which is probably not what you meant.
40Use braces to disambiguate, as in C<"This is ${owner}'s house">.
a0d0e21e 41
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42Packages may themselves contain package separators, as in
43C<$OUTER::INNER::var>. This implies nothing about the order of
44name lookups, however. There are no relative packages: all symbols
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45are either local to the current package, or must be fully qualified
46from the outer package name down. For instance, there is nowhere
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47within package C<OUTER> that C<$INNER::var> refers to
48C<$OUTER::INNER::var>. It would treat package C<INNER> as a totally
49separate global package.
50
51Only identifiers starting with letters (or underscore) are stored
52in a package's symbol table. All other symbols are kept in package
53C<main>, including all punctuation variables, like $_. In addition,
54when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
55ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
56even when used for other purposes than their built-in one. If you
57have a package called C<m>, C<s>, or C<y>, then you can't use the
58qualified form of an identifier because it would be instead interpreted
59as a pattern match, a substitution, or a transliteration.
60
61Variables beginning with underscore used to be forced into package
a0d0e21e 62main, but we decided it was more useful for package writers to be able
cb1a09d0 63to use leading underscore to indicate private variables and method names.
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64$_ is still global though. See also
65L<perlvar/"Technical Note on the Syntax of Variable Names">.
a0d0e21e 66
19799a22 67C<eval>ed strings are compiled in the package in which the eval() was
a0d0e21e 68compiled. (Assignments to C<$SIG{}>, however, assume the signal
748a9306 69handler specified is in the C<main> package. Qualify the signal handler
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70name if you wish to have a signal handler in a package.) For an
71example, examine F<perldb.pl> in the Perl library. It initially switches
72to the C<DB> package so that the debugger doesn't interfere with variables
19799a22 73in the program you are trying to debug. At various points, however, it
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74temporarily switches back to the C<main> package to evaluate various
75expressions in the context of the C<main> package (or wherever you came
76from). See L<perldebug>.
77
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78The special symbol C<__PACKAGE__> contains the current package, but cannot
79(easily) be used to construct variables.
80
5f05dabc 81See L<perlsub> for other scoping issues related to my() and local(),
f102b883 82and L<perlref> regarding closures.
cb1a09d0 83
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84=head2 Symbol Tables
85
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86The symbol table for a package happens to be stored in the hash of that
87name with two colons appended. The main symbol table's name is thus
5803be0d 88C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
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89package mentioned earlier is named C<%OUTER::INNER::>.
90
91The value in each entry of the hash is what you are referring to when you
92use the C<*name> typeglob notation. In fact, the following have the same
93effect, though the first is more efficient because it does the symbol
94table lookups at compile time:
a0d0e21e 95
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96 local *main::foo = *main::bar;
97 local $main::{foo} = $main::{bar};
a0d0e21e 98
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99(Be sure to note the B<vast> difference between the second line above
100and C<local $main::foo = $main::bar>. The former is accessing the hash
101C<%main::>, which is the symbol table of package C<main>. The latter is
102simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
103the same package.)
104
a0d0e21e 105You can use this to print out all the variables in a package, for
4375e838 106instance. The standard but antiquated F<dumpvar.pl> library and
19799a22 107the CPAN module Devel::Symdump make use of this.
a0d0e21e 108
cb1a09d0 109Assignment to a typeglob performs an aliasing operation, i.e.,
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110
111 *dick = *richard;
112
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113causes variables, subroutines, formats, and file and directory handles
114accessible via the identifier C<richard> also to be accessible via the
115identifier C<dick>. If you want to alias only a particular variable or
19799a22 116subroutine, assign a reference instead:
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117
118 *dick = \$richard;
119
5a964f20 120Which makes $richard and $dick the same variable, but leaves
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121@richard and @dick as separate arrays. Tricky, eh?
122
cb1a09d0 123This mechanism may be used to pass and return cheap references
5803be0d 124into or from subroutines if you don't want to copy the whole
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125thing. It only works when assigning to dynamic variables, not
126lexicals.
cb1a09d0 127
5a964f20 128 %some_hash = (); # can't be my()
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129 *some_hash = fn( \%another_hash );
130 sub fn {
131 local *hashsym = shift;
132 # now use %hashsym normally, and you
133 # will affect the caller's %another_hash
134 my %nhash = (); # do what you want
5f05dabc 135 return \%nhash;
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136 }
137
5f05dabc 138On return, the reference will overwrite the hash slot in the
cb1a09d0 139symbol table specified by the *some_hash typeglob. This
c36e9b62 140is a somewhat tricky way of passing around references cheaply
5803be0d 141when you don't want to have to remember to dereference variables
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142explicitly.
143
19799a22 144Another use of symbol tables is for making "constant" scalars.
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145
146 *PI = \3.14159265358979;
147
bc8df162 148Now you cannot alter C<$PI>, which is probably a good thing all in all.
5a964f20 149This isn't the same as a constant subroutine, which is subject to
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150optimization at compile-time. A constant subroutine is one prototyped
151to take no arguments and to return a constant expression. See
152L<perlsub> for details on these. The C<use constant> pragma is a
5a964f20 153convenient shorthand for these.
cb1a09d0 154
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155You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
156package the *foo symbol table entry comes from. This may be useful
5a964f20 157in a subroutine that gets passed typeglobs as arguments:
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158
159 sub identify_typeglob {
160 my $glob = shift;
161 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
162 }
163 identify_typeglob *foo;
164 identify_typeglob *bar::baz;
165
166This prints
167
168 You gave me main::foo
169 You gave me bar::baz
170
19799a22 171The C<*foo{THING}> notation can also be used to obtain references to the
5803be0d 172individual elements of *foo. See L<perlref>.
55497cff 173
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174Subroutine definitions (and declarations, for that matter) need
175not necessarily be situated in the package whose symbol table they
176occupy. You can define a subroutine outside its package by
177explicitly qualifying the name of the subroutine:
178
179 package main;
180 sub Some_package::foo { ... } # &foo defined in Some_package
181
182This is just a shorthand for a typeglob assignment at compile time:
183
184 BEGIN { *Some_package::foo = sub { ... } }
185
186and is I<not> the same as writing:
187
188 {
189 package Some_package;
190 sub foo { ... }
191 }
192
193In the first two versions, the body of the subroutine is
194lexically in the main package, I<not> in Some_package. So
195something like this:
196
197 package main;
198
199 $Some_package::name = "fred";
200 $main::name = "barney";
201
202 sub Some_package::foo {
203 print "in ", __PACKAGE__, ": \$name is '$name'\n";
204 }
205
206 Some_package::foo();
207
208prints:
209
210 in main: $name is 'barney'
211
212rather than:
213
214 in Some_package: $name is 'fred'
215
216This also has implications for the use of the SUPER:: qualifier
217(see L<perlobj>).
218
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219=head2 Package Constructors and Destructors
220
7d981616 221Four special subroutines act as package constructors and destructors.
7d30b5c4 222These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The
7d981616 223C<sub> is optional for these routines.
a0d0e21e 224
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225A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
226it is completely defined, even before the rest of the containing file
227is parsed. You may have multiple C<BEGIN> blocks within a file--they
228will execute in order of definition. Because a C<BEGIN> block executes
229immediately, it can pull in definitions of subroutines and such from other
230files in time to be visible to the rest of the file. Once a C<BEGIN>
231has run, it is immediately undefined and any code it used is returned to
232Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
a0d0e21e 233
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234An C<END> subroutine is executed as late as possible, that is, after
235perl has finished running the program and just before the interpreter
236is being exited, even if it is exiting as a result of a die() function.
237(But not if it's polymorphing into another program via C<exec>, or
238being blown out of the water by a signal--you have to trap that yourself
239(if you can).) You may have multiple C<END> blocks within a file--they
240will execute in reverse order of definition; that is: last in, first
241out (LIFO). C<END> blocks are not executed when you run perl with the
db517d64 242C<-c> switch, or if compilation fails.
a0d0e21e 243
19799a22 244Inside an C<END> subroutine, C<$?> contains the value that the program is
c36e9b62 245going to pass to C<exit()>. You can modify C<$?> to change the exit
19799a22 246value of the program. Beware of changing C<$?> by accident (e.g. by
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247running something via C<system>).
248
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249Similar to C<BEGIN> blocks, C<INIT> blocks are run just before the
250Perl runtime begins execution, in "first in, first out" (FIFO) order.
251For example, the code generators documented in L<perlcc> make use of
252C<INIT> blocks to initialize and resolve pointers to XSUBs.
253
7d30b5c4 254Similar to C<END> blocks, C<CHECK> blocks are run just after the
4f25aa18 255Perl compile phase ends and before the run time begins, in
7d30b5c4 256LIFO order. C<CHECK> blocks are again useful in the Perl compiler
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257suite to save the compiled state of the program.
258
19799a22 259When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
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260C<END> work just as they do in B<awk>, as a degenerate case.
261Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>
262switch for a compile-only syntax check, although your main code
263is not.
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264
265=head2 Perl Classes
266
19799a22 267There is no special class syntax in Perl, but a package may act
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268as a class if it provides subroutines to act as methods. Such a
269package may also derive some of its methods from another class (package)
19799a22 270by listing the other package name(s) in its global @ISA array (which
5a964f20 271must be a package global, not a lexical).
4633a7c4 272
f102b883 273For more on this, see L<perltoot> and L<perlobj>.
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274
275=head2 Perl Modules
276
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277A module is just a set of related functions in a library file, i.e.,
278a Perl package with the same name as the file. It is specifically
279designed to be reusable by other modules or programs. It may do this
280by providing a mechanism for exporting some of its symbols into the
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281symbol table of any package using it. Or it may function as a class
282definition and make its semantics available implicitly through
283method calls on the class and its objects, without explicitly
4375e838 284exporting anything. Or it can do a little of both.
a0d0e21e 285
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286For example, to start a traditional, non-OO module called Some::Module,
287create a file called F<Some/Module.pm> and start with this template:
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288
289 package Some::Module; # assumes Some/Module.pm
290
291 use strict;
9f1b1f2d 292 use warnings;
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293
294 BEGIN {
295 use Exporter ();
77ca0c92 296 our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
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297
298 # set the version for version checking
299 $VERSION = 1.00;
300 # if using RCS/CVS, this may be preferred
301 $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
302
303 @ISA = qw(Exporter);
304 @EXPORT = qw(&func1 &func2 &func4);
305 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
306
307 # your exported package globals go here,
308 # as well as any optionally exported functions
309 @EXPORT_OK = qw($Var1 %Hashit &func3);
310 }
77ca0c92 311 our @EXPORT_OK;
9607fc9c 312
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313 # exported package globals go here
314 our $Var1;
315 our %Hashit;
316
9607fc9c 317 # non-exported package globals go here
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318 our @more;
319 our $stuff;
9607fc9c 320
c2611fb3 321 # initialize package globals, first exported ones
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322 $Var1 = '';
323 %Hashit = ();
324
325 # then the others (which are still accessible as $Some::Module::stuff)
326 $stuff = '';
327 @more = ();
328
329 # all file-scoped lexicals must be created before
330 # the functions below that use them.
331
332 # file-private lexicals go here
333 my $priv_var = '';
334 my %secret_hash = ();
335
336 # here's a file-private function as a closure,
337 # callable as &$priv_func; it cannot be prototyped.
338 my $priv_func = sub {
339 # stuff goes here.
340 };
341
342 # make all your functions, whether exported or not;
343 # remember to put something interesting in the {} stubs
344 sub func1 {} # no prototype
345 sub func2() {} # proto'd void
346 sub func3($$) {} # proto'd to 2 scalars
347
348 # this one isn't exported, but could be called!
349 sub func4(\%) {} # proto'd to 1 hash ref
350
351 END { } # module clean-up code here (global destructor)
4633a7c4 352
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353 ## YOUR CODE GOES HERE
354
355 1; # don't forget to return a true value from the file
356
357Then go on to declare and use your variables in functions without
358any qualifications. See L<Exporter> and the L<perlmodlib> for
359details on mechanics and style issues in module creation.
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360
361Perl modules are included into your program by saying
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362
363 use Module;
364
365or
366
367 use Module LIST;
368
369This is exactly equivalent to
370
5a964f20 371 BEGIN { require Module; import Module; }
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372
373or
374
5a964f20 375 BEGIN { require Module; import Module LIST; }
a0d0e21e 376
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377As a special case
378
379 use Module ();
380
381is exactly equivalent to
382
5a964f20 383 BEGIN { require Module; }
cb1a09d0 384
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385All Perl module files have the extension F<.pm>. The C<use> operator
386assumes this so you don't have to spell out "F<Module.pm>" in quotes.
387This also helps to differentiate new modules from old F<.pl> and
388F<.ph> files. Module names are also capitalized unless they're
389functioning as pragmas; pragmas are in effect compiler directives,
390and are sometimes called "pragmatic modules" (or even "pragmata"
391if you're a classicist).
a0d0e21e 392
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393The two statements:
394
395 require SomeModule;
396 require "SomeModule.pm";
397
398differ from each other in two ways. In the first case, any double
399colons in the module name, such as C<Some::Module>, are translated
400into your system's directory separator, usually "/". The second
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401case does not, and would have to be specified literally. The other
402difference is that seeing the first C<require> clues in the compiler
403that uses of indirect object notation involving "SomeModule", as
404in C<$ob = purge SomeModule>, are method calls, not function calls.
405(Yes, this really can make a difference.)
406
407Because the C<use> statement implies a C<BEGIN> block, the importing
408of semantics happens as soon as the C<use> statement is compiled,
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409before the rest of the file is compiled. This is how it is able
410to function as a pragma mechanism, and also how modules are able to
19799a22 411declare subroutines that are then visible as list or unary operators for
a0d0e21e 412the rest of the current file. This will not work if you use C<require>
19799a22 413instead of C<use>. With C<require> you can get into this problem:
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414
415 require Cwd; # make Cwd:: accessible
54310121 416 $here = Cwd::getcwd();
a0d0e21e 417
5f05dabc 418 use Cwd; # import names from Cwd::
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419 $here = getcwd();
420
421 require Cwd; # make Cwd:: accessible
422 $here = getcwd(); # oops! no main::getcwd()
423
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424In general, C<use Module ()> is recommended over C<require Module>,
425because it determines module availability at compile time, not in the
426middle of your program's execution. An exception would be if two modules
427each tried to C<use> each other, and each also called a function from
428that other module. In that case, it's easy to use C<require>s instead.
cb1a09d0 429
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430Perl packages may be nested inside other package names, so we can have
431package names containing C<::>. But if we used that package name
5803be0d 432directly as a filename it would make for unwieldy or impossible
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433filenames on some systems. Therefore, if a module's name is, say,
434C<Text::Soundex>, then its definition is actually found in the library
435file F<Text/Soundex.pm>.
436
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437Perl modules always have a F<.pm> file, but there may also be
438dynamically linked executables (often ending in F<.so>) or autoloaded
5803be0d 439subroutine definitions (often ending in F<.al>) associated with the
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440module. If so, these will be entirely transparent to the user of
441the module. It is the responsibility of the F<.pm> file to load
442(or arrange to autoload) any additional functionality. For example,
443although the POSIX module happens to do both dynamic loading and
5803be0d 444autoloading, the user can say just C<use POSIX> to get it all.
a0d0e21e 445
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446=head2 Making your module threadsafe
447
448Perl has since 5.6.0 support for a new type of threads called
449interpreter threads. These threads can be used explicitly and implicitly.
450
451Ithreads work by cloning the data tree so that no data is shared
452between different threads. These threads can be used using the threads
453module or by doing fork() on win32 (fake fork() support). When a thread is
454cloned all perl data is cloned, however non perl data cannot be cloned.
455Perl after 5.7.2 has support for the C<CLONE> keyword. C<CLONE> will be
456executed once for every package that has it defined (or inherits it).
457It will be called in the context of the new thread, so all modifications
458are made in the new area.
459
460If you want to CLONE all objects you will need to keep track of them per
461package. This is simply done using a hash and Scalar::Util::weaken().
462
f102b883 463=head1 SEE ALSO
cb1a09d0 464
f102b883 465See L<perlmodlib> for general style issues related to building Perl
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466modules and classes, as well as descriptions of the standard library
467and CPAN, L<Exporter> for how Perl's standard import/export mechanism
468works, L<perltoot> and L<perltootc> for an in-depth tutorial on
469creating classes, L<perlobj> for a hard-core reference document on
470objects, L<perlsub> for an explanation of functions and scoping,
471and L<perlxstut> and L<perlguts> for more information on writing
472extension modules.