This is because use lib $mod_path is evaluated during compile time (it's equivalent to BEGIN { require lib; lib->import($mod_path) ; } - but the assignement to $mod_path is done during runtime - in effect you are calling lib->import with an empty list as argument

I modified my lib.pm to produce a little message every time import is called:

my @mod_path = '/home/ripenapp/perl_mods';
use lib @mod_path;
use Email::Valid;
__END__
tomte@librics-tomte ~/scratch/test $ perl test.pl
Importing:
[download]

Your first version:

use lib '/home/tanger/www/mods';
use Email::Valid;
__END__
tomte@librics-tomte ~/scratch/test $ perl test.pl
Importing: /home/tanger/www/mods
[download]