Best practice is to always use strictures, declare variables to minimize their scope and initialize scalars with a useful value at declaration time (except where conditional initialization is required).

Never initialize a scalar variable with a 'junk' value just to shut up warnings! The warnings give you a heads up about bugs in your code.

undef is a vitally important variable state that allows a lot of sanity checking that otherwise is difficult to achieve. Strictures are your friend, perhaps even a best friend.

Note that arrays and hashes don't need to be "initialized". They start empty in any case so something of the form my @array = (); is redundant.