From what I can see from the source of Algorithm::LUHN nothing is exported by default: 
@EXPORT    = qw//;
@EXPORT_OK = qw/check_digit is_valid valid_chars/;

[download]

so if you wanna import is_valid you need to tell explicitly.

see also How to Export

EDIT

> 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.

Cheers Rolf

In reply to Re: Difference between use Module::Name and use Module::Name qw /method1 method2/ by LanX
in thread Difference between use Module::Name and use Module::Name qw /method1 method2/ by Doctrin

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