in reply to Some "combined assignment operators" are exempt from "uninitialized value" warning. Is it documented anywhere?

You don't get a warning when undef is interpreted as the identity element of the operator. Or, in other words, when the following pattern isn't useless

my $value; while (condition) # or for loop { $value <op>= something; }
For example:
my $sum; $sum += $_ for @nums;
my $valid; while (<DATA>) { chomp; next if /INVALID/; $valid .= $_; }
And not setting the variable before the loop let's you tell if there were no valid lines at all, or if the only valid data was empty strings (or $sum will be undef rather than 0 if there are no numbers).

On the other hand you can use whatever values you want, this will never yield anything but 0:

my $product; $product *= $_ for @nums;
This can only be a mistake.

So this would tend towards "this a right thing to do", but I couldn't find the documentation either.

Edit: that was full of typos. I wrote "under" for "undef", "is" for "if" and "identify" for "identity". I hope I got them all...