As for #2...no you don't get the proper results. What you're actually getting is: 012345678undef but of course undef doesn't print. :)

Watch this. It's your code with two small modifications:

use warnings;
use strict;
my @foo = ( 1 , 3 , 5 , 7 );
my @bar = (0, 2 , 4 , 6 , 8 );
my @foobar= map { $_, shift @foo } @bar;
print STDOUT join(',', @foobar);
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When you run this, notice that the list is going to have a trailing ",". That's because join() knows there's an element after the 8 (undef!) and makes a slot for it. And notice the error changed, its now an uninit value in join.