perlfaq3 shows how to do this with the ExtUtils::Installed module in "How do I find which modules are installed on my system?". There is also an example which uses the File::Find module, and a hint to look at the Module::CoreList module to identify core modules in different versions of perl.
Note: this particular FAQ is present in the docs for perl 5.8.7, but not in the docs for 5.6.1.
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