Inside your Makefile.PL, you have a WriteMakefile function. That function takes a list of key-value pairs which it will use to figure out what's going on and what to do when a user tries to install the module.

Inside that function, you should see an entry for PREREQ_PM => {}. This part allows you to specify all of the prerequisite modules along with their minimum versions. For example, this little snippet requires Foo::Bar version 1.02 or greater, Baz::Quux 2.70 or greater, and any version of CGI.pm.

WriteMakefile(
#...
    PREREQ_PM => {
        Foo::Bar => 1.02,
        Baz::Quux => 2.70,
        CGI       => 0,
        }
#...
);
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Tools such as CPAN.pm and CPANPLUS can use this information to automatically install and update dependencies.

If you'd like to discover the module versions that you have installed, you can use a quick command line (among hundreds of other ways):

$ perl -MCGI -le 'print CGI->VERSION' 
3.04
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If you have the latest version (1.051.51) of my cpan(1) utility, you can use the -D switch to get module details. This is handy not only to check your installed version of the module, but to see what CPAN thinks is the current version.

$ cpan -D CGI
CGI
---------------------------------------------------------------------
        (no description)
        L/LD/LDS/CGI.pm-3.15.tar.gz
        /usr/local/lib/perl5/5.8.4/CGI.pm
        Installed: 3.04
        CPAN:      3.15  Not up to date
        Lincoln D. Stein (LDS)
        lstein@cshl.org
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Good luck :)