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
while (condition) # or for loop
$value <op>= something;
$sum += $_ for @nums;
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:
$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...