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;
}
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For example:

my $sum; 
$sum += $_ for @nums;
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or

my $valid;
while (<DATA>)
{
  chomp;
  next if /INVALID/;
  $valid .= $_;
}
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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;
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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...