The answer is always going to be: It depends. Upon many things including what you subsequently do with the variables.

One trivial test each of ''. and 0+ shows that in these particular cases, the former costs little, and the latter saves a lot:

cmpthese -1,{
    a => q[ my $s = join '', map ''.$_, 1 .. 1e5 ],
    b => q[ my $s = join '', map    $_, 1 .. 1e5 ], 
};;
    Rate    a    b
a 12.3/s   -- -18%
b 14.9/s  21%   --

cmpthese -1,{
    a => q[ my $s = sum map 0+$_, '0001' .. '1000'],
    b => q[ my $s = sum map   $_, '0001' .. '1000'], 
};;
    Rate    b    a
b 1452/s   -- -76%
a 6056/s 317%   --
[download]

I guess whether you think either or both are important will depend upon whether your programs spend most of their time sitting around waiting for someone to click something, or doing real work :)

While you're on the subject you ought to check out Damian Conway's Contextual::Return. It lets you specify explicitly (rather than through obscure tricks) what behavior subs will take under various contexts. The control is very fine grained, even letting you specify boolean behavior, numeric behavior, and so on.

Documented in its own POD, as well as Perl Best Practices, chapter 9.

It's probably not useful as a micro-optimization. But 97% of the time you wouldn't want to care about such things anyway. :) And when you run into one of those 3% situations, be sure to document well why an obscure trick has been applied.

Have you ever looked inside Contextual::Return?

Or measured the affect it has on the performance of your subroutines and methods?

As of Perl 5.14, B::Concise shows that the optree contains the appropriate add and concat ops, and the optimizer does not optimize them away.

Just doing a simple math or concantation on a variable, is simpler than opening up some object, and doing a change in it. Just from basic principles of information.