As was recently explained to me in a similar node, use module; statements are enclosed within an implicit BEGIN { } block.

With regard to your second question, there is no difference between the relative placement of the use statement as it is implicitly located within a BEGIN { } block, rendering the two codes identical.

The answer to your third question can be found in perlfunc under use, but suffice it to say the list corresponds to the module's exports, thus allowing selective importation of the module.