How do I write a new sub in a git cloned CPAN module and test it bypassing the Original Module

yysachinyy has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

I git cloned DateTime module from cpan. I already had a DateTime installed. I added a new method in DateTime.pm and worte a test for a new method added. Now when I run the script it uses the already installed DateTime module and not the one I have modified and get error "undefined sub new_subname".

I tried setting PERL5LIB and use lib pragma so that the test script finds modifed cloned DateTime.pm but din't help.

I would like to know how Perl gurus/monks solve this issue and contribute to CPAN Module.
Thanks.

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