Even variables that you do not declare to be in any package are by default in the "mother of all packages", main. So when you write "$x=10;" perl really sees it as

$main::x=10;
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Since everything is based off of main, the examples above can also be written as:

$main::A::v
and
$main::B::v
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(Both ways are 'correct', although the previous way is actually preferred.)

Unless you are creating your own packages, you don't have to worry about namespace or about using the double colon identifier - perl does all of that behind your back. But it is a nice feeling to know that perl is back there, keeping track of everything for you. :)