From @Grinnz

On Fri, Dec 22, 2017 at 4:04 PM, James E Keenan via RT <
perlbug-followup@perl.org> wrote:

On Fri, 22 Dec 2017 17:39:52 GMT, public@khwilliamson.com wrote:

I am posting this for Dan Book, who is having trouble getting email to
perlbug through.

The build options and version of perl is immaterial here.

1. When called in scalar context in a while loop, the condition is
wrapped
implicitly in a 'defined' check, much like with 'readline'.

e.g. `while (my $key = each %hash) { ... }` becomes
`while (defined(my $key = each %hash)) { ... }`, and
`while (each %hash) { ... }` becomes `while (defined($_ = each %hash)) {
... }`
(on 5.18+)

2. The iterator is not only shared and reset by calling 'keys' or
'values', but
also by other calls to 'each' on the same hash or array, and by
accessing the
hash in list context (such as for assignment to another structure, or
passing
to a function) - this last one only applies to hashes, not arrays.

Can you provide examples where failing to understand these concepts would
trip someone up?

For 1, someone might assume that a `while (my $key = each %hash)` loop will
terminate if it encounters an empty string or '0' key, or be confused when
it doesn't, since there's no documentation similar to that for `while (my
$line = <$fh>)` which shares the behavior.

For 2, it would not be apparent that the uses I listed would be destructive
to such an iteration. Such as:

while (my ($k, $v) = each %hash) {
my %copy = %hash;
}

while (my ($k, $v) = each %hash) {
while (my ($inner_k, $inner_v) = each %hash) {
...
}
}

-Dan

The 'each' function documentation is missing some information. #16334

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