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Module Version: 0.28   Source  


only - Load specific module versions; Install many


    # Install version 0.30 of MyModule
    cd MyModule-0.30
    perl Makefile.PL
    make test
    perl -Monly=install    # substitute for 'make install' 
    perl -Monly=install - version=0.33 versionlib=/home/ingy/perlmods
    # Only use MyModule version 0.30
    use only MyModule => 0.30;

    # Only use MyModule if version is between 0.30 and 0.50
    # but not 0.36; or if version is >= to 0.55.
    use only MyModule => '0.30-0.50 !0.36 0.55-', qw(:all);

    # Don't export anything!
    use only MyModule => 0.30, [];

    # Version dependent arguments
    use only MyModule =>
        [ '0.20-0.27', qw(f1 f2 f3 f4) ],
        [ '0.30-',     qw(:all) ];

    # Override versionlib
    use only {versionlib => '/home/ingy/perlmods'},
        MyModule => 0.33;
    # Override versionlib globally
    use only {versionlib => '/home/ingy/perlmods'};
    use only MyModule => 0.33;

    # Object Oriented Interface
    use only;
    $only = only->new;
    require MyModule;


    # Note: <angle brackets> mean "optional".

    # To load a specific module

    # To set options
    use only < { OPTIONS HASH } >, MODULE => 'CONDITION SPEC';

    # To set options globally
    use only < { OPTIONS HASH } >;

    # For multiple argument sets
    use only MODULE => 
        ['CONDITION SPEC 1' <, ARGUMENTS1>],
        ['CONDITION SPEC 2' <, ARGUMENTS2>],

    # To install an alternate version of a module
    perl -Monly=install <- ARGUMENTS>        # instead of 'make install'


The facility allows you to load a MODULE only if it satisfies a given CONDITION. Normally that condition is a version. If you just specify a single version, 'only' will only load the module matching that version. If you specify multiple versions, the module can be any of those versions. See below for all the different conditions you can use with only. will also allow you to load a particular version of a module, when many versions of the same module are installed. See below for instructions on how to easily install many different versions of the same module.


A condition specification is a single string containing a list of zero or more conditions. The list of conditions is separated by spaces. Each condition can take one of the following forms:

The search works by searching the version-lib directories (found in only::config) for a module that meets the condition specification. If more than one version is found, the highest version is used. If no module meets the specification, then a normal @INC style require is performed.

If the condition is a subroutine reference, that subroutine will be called and passed an only object. If the subroutine returns a false value, the program will die. See below for a list of public methods that may be used upon the only object.


All of the arguments following the CONDITION specification, will be passed to the module being loaded.

Normally you can pass an empty list to use to turn off Exporting. To do this with only, use an empty array ref.

    use only MyModule => '0.30';       # Default exporting
    use only MyModule => '0.30', [];   # No exporting
    use only MyModule => '0.30', qw(export list);  # Specific export

If you need pass different arguments depending on which version is used, simply wrap each condition spec and arguments with an array ref.

    use only MyModule =>
        [ '0.20-0.27', qw(f1 f2 f3 f4) ],
        [ '0.30-',     qw(:all) ];


Options to only are specified as a hash reference placed before the module name. If there is no module name, the options become global, and affect all other calls to only (even ones from other modules, so be aware).

Currently, the only option is versionlib.

Sometimes you need to tell only to use a specific version library to load from. Use the versionlib option to do this.

    use only { versionlib => '/home/ingy/modules' },
        MyModule => 0.33;


The module also has a facility for installing more than one version of a particular module. Using this facility you can install an older version of a module and use it with the 'use only' syntax.

It works like this; when installing a module, do the familiar:

    perl Makefile.PL
    make test

But instead of make install, do this:

    perl -Monly=install

This will attempt to determine what version the module should be installed under. In some cases you may need to specify the version yourself. Do the following:

    perl -Monly=install - version=0.55

By default, everything will be installed in versionlib directory stored in only::config. To override the installation location, do this:

    perl -Monly=install - versionlib=/home/ingy/modules

NOTE: Also works with Module::Build style modules.

NOTE: The perl you use for this must be the same perl as the one used to do perl Makefile.PL or perl Build.PL. While this seems obvious, you may run into problems with sudo perl -Monly=install, since the root account may have a different perl in its path. If this happens, just use the full path to your perl.

Installing with Module::Build

When installing modules distributed with Module::Build, you can use the following commands to install into version specific libraries:

    perl Build.PL
    ./Build versioninstall

For overrides:

    perl Build.PL version=1.23 versionlib=/home/ingy/modules
    ./Build versioninstall

NOTE: The Module::Build verion install does not suffer from the same sudo problem outlined above. Module::Build remembers the original perl path.


When you install the only module, you can tell it where to install alternate versions of modules. These paths get stored into only::config. The default location to install things is parallel to your sitelib. For instance if your sitelib was:


only would default to:


This keeps your normal install trees free from any potential complication with version modules.

If you install version 0.24 and 0.26 of MyModule and version 0.26 of Your::Module, they will end up here:


HOW IT WORKS ^ is kind of like on Koolaid! Instead of adding a search path to @INC, it adds a search object to @INC. This object is actually the object itself. The object keeps track of all of the modules related to a given module distribution installation, and takes responsibility for loading those modules. This is very important because if you say:

    use only Goodness => '0.23';

and then later:

    require Goodness::Gracious;

you want to be sure that the correct version of the second module gets loaded. Especially when another module is doing the loading.


only is implemented internally using Object Oriented Programming. You yourself can also make use of only objects directly in your program. Instead of saying something like this:

    use only MyModule => '0.30', qw(foo bar);

You could say:

    my $only;
    BEGIN {
        $only = only->new;
    use MyModule qw(foo bar);

The cool thing here is that we just used a normal use statement to load a particular module.

This gives you more control and you may be able to do some interesting stuff this way.

The following sections detail the Object Oriented API.

Class Methods

There are three class methods available:

Object Methods

All of the following methods return themselves when used as store-accessors. This lets you chain calls together:


When used as fetch-accessors they, of course, return their values.


The module loads a module by the following process:

 1) Look for the highest suitable version of the module in the version
    libraries specified in only::config.
 2) Do a normal require() of the module, and check to make sure the 
    version is in the range specified.

It is important to understand that the versions used in these two different steps come from different places and might not be the same.

In the first step the version used is the version of the distribution that the module was installed from. This is grepped out of the Makefile and saved as metadata for that module.

In the second step, the version is taken from $VERSION of that module. This is the same process used when you do something like:

     use MyModule '0.50';

Unfortunately, there is no way to know what the distribution version is for a normally installed module.

Fortunately, $VERSION is usually the same as the distribution version. That's because the popular VERSION_FROM Makefile.PL option makes it happen. Authors are encouraged to use this option.

The conclusion here is that usually gets things right. Always check %INC, if you suspect that the wrong versions are being pulled in. If this happens, use more 'use only' statements to pull in the right versions.

One failsafe solution is to make sure that all module versions in question are installed into the version libraries.


You can't do that! Are you crazy? Well I am. I can't do this yet but I'd really like to. I'm working on it. If you have ideas on how this might be accomplished, send me an email. If you don't have a good idea, send me some coffee.



Brian Ingerson <>


Copyright (c) 2003. Brian Ingerson. All rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


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