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Gurusamy Sarathy


perlmod - Perl modules (packages and symbol tables)



Perl provides a mechanism for alternative namespaces to protect packages from stomping on each other's variables. In fact, there's really no such thing as a global variable in Perl (although some identifiers default to the main package instead of the current one). The package statement declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block, eval, sub, or end of file, whichever comes first (the same scope as the my() and local() operators). All further unqualified dynamic identifiers will be in this namespace. A package statement only affects dynamic variables--including those you've used local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file to be included by the require or use operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package is assumed. That is, $::sail is equivalent to $main::sail.


Packages may be nested inside other packages: $OUTER::INNER::var. This implies nothing about the order of name lookups, however. All symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere within package OUTER that $INNER::var refers to $OUTER::INNER::var. It would treat package INNER as a totally separate global package.

Only identifiers starting with letters (or underscore) are stored in a package's symbol table. All other symbols are kept in package main, including all of the punctuation variables like $_. In addition, when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are forced to be in package main, even when used for other purposes than their builtin one. Note also that, if you have a package called m, s, or y, then you can't use the qualified form of an identifier because it will be interpreted instead as a pattern match, a substitution, or a transliteration.





Symbol Tables




You can use this to print out all the variables in a package, for instance. The standard dumpvar.pl library and the CPAN module Devel::Symdump make use of this.

Assignment to a typeglob performs an aliasing operation, i.e.,

    *dick = *richard;

causes variables, subroutines, formats, and file and directory handles accessible via the identifier richard also to be accessible via the identifier dick. If you want to alias only a particular variable or subroutine, you can assign a reference instead:

    *dick = \$richard;



    %some_hash = ();                    # can't be my()
    *some_hash = fn( \%another_hash );
    sub fn {
        local *hashsym = shift;
        # now use %hashsym normally, and you
        # will affect the caller's %another_hash
        my %nhash = (); # do what you want
        return \%nhash;



    *PI = \3.14159265358979;



    sub identify_typeglob {
        my $glob = shift;
    identify_typeglob *foo;
    identify_typeglob *bar::baz;

This prints

    You gave me main::foo
    You gave me bar::baz


Package Constructors and Destructors

There are two special subroutine definitions that function as package constructors and destructors. These are the BEGIN and END routines. The sub is optional for these routines.


An END subroutine is executed as late as possible, that is, when the interpreter is being exited, even if it is exiting as a result of a die() function. (But not if it's polymorphing into another program via exec, or being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO).

Inside an END subroutine, $? contains the value that the script is going to pass to exit(). You can modify $? to change the exit value of the script. Beware of changing $? by accident (e.g. by running something via system).

Note that when you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk, as a degenerate case. As currently implemented (and subject to change, since its inconvenient at best), both BEGIN and END blocks are run when you use the -c switch for a compile-only syntax check, although your main code is not.

Perl Classes

There is no special class syntax in Perl, but a package may function as a class if it provides subroutines to act as methods. Such a package may also derive some of its methods from another class (package) by listing the other package name in its global @ISA array (which must be a package global, not a lexical).

For more on this, see perltoot and perlobj.

Perl Modules

A module is just a package that is defined in a library file of the same name, and is designed to be reusable. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicit exportation of any symbols. Or it can do a little of both.

For example, to start a normal module called Some::Module, create a file called Some/Module.pm and start with this template:

    package Some::Module;  # assumes Some/Module.pm

    use strict;

    BEGIN {
        use Exporter   ();

        # set the version for version checking
        $VERSION     = 1.00;
        # if using RCS/CVS, this may be preferred

        @ISA         = qw(Exporter);

        # your exported package globals go here,
        # as well as any optionally exported functions
    use vars      @EXPORT_OK;

    # non-exported package globals go here
    use vars      qw(@more $stuff);

    # initalize package globals, first exported ones
    $Var1   = '';
    %Hashit = ();

    $stuff  = '';
    @more   = ();

    # all file-scoped lexicals must be created before
    # the functions below that use them.

    # file-private lexicals go here
    my $priv_var    = '';
    my %secret_hash = ();

    # here's a file-private function as a closure,
    # callable as &$priv_func;  it cannot be prototyped.
    my $priv_func = sub {
        # stuff goes here.

    # make all your functions, whether exported or not;
    # remember to put something interesting in the {} stubs
    sub func1      {}    # no prototype
    sub func2()    {}    # proto'd void

    # this one isn't exported, but could be called!



Perl modules are included into your program by saying

    use Module;


    use Module LIST;

This is exactly equivalent to

    BEGIN { require Module; import Module; }


    BEGIN { require Module; import Module LIST; }

As a special case

    use Module ();

is exactly equivalent to

    BEGIN { require Module; }

All Perl module files have the extension .pm. use assumes this so that you don't have to spell out "Module.pm" in quotes. This also helps to differentiate new modules from old .pl and .ph files. Module names are also capitalized unless they're functioning as pragmas, "Pragmas" are in effect compiler directives, and are sometimes called "pragmatic modules" (or even "pragmata" if you're a classicist).

The two statements:

    require SomeModule;
    require "SomeModule.pm";            


Because the use statement implies a BEGIN block, the importation of semantics happens at the moment the use statement is compiled, before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to declare subroutines that are then visible as list operators for the rest of the current file. This will not work if you use require instead of use. With require you can get into this problem:

    require Cwd;                # make Cwd:: accessible
    $here = Cwd::getcwd();

    use Cwd;                    # import names from Cwd::
    $here = getcwd();

    require Cwd;                # make Cwd:: accessible



Perl modules always have a .pm file, but there may also be dynamically linked executables or autoloaded subroutine definitions associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the .pm file to load (or arrange to autoload) any additional functionality. The POSIX module happens to do both dynamic loading and autoloading, but the user can say just use POSIX to get it all.

For more information on writing extension modules, see perlxstut and perlguts.


See perlmodlib for general style issues related to building Perl modules and classes as well as descriptions of the standard library and CPAN, Exporter for how Perl's standard import/export mechanism works, perltoot for an in-depth tutorial on creating classes, perlobj for a hard-core reference document on objects, and perlsub for an explanation of functions and scoping.