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From what I can see from the source of Algorithm::LUHN nothing is exported by default:
so if you wanna import is_valid you need to tell explicitly.
see also How to Export
> Is it somehow better to use the second statement if I use only "is_valid" method in my code? Maybe, memory issues or something?
No! No significant memory problems, importing is a kind of aliasing between namespaces.
But you have to care about potential namespace polution, see Selecting What To Export.
Please note that you don't necessarily need to import is_valid, otherwise can also address Algorithm::LUHN::is_valid directly.
In reply to Re: Difference between use Module::Name and use Module::Name qw /method1 method2/