It means you've already started iterating over the hash.

sub _split_options {
    my ($opts) = @_;

    while (my ($k, $v) = each %$opts) {
        print "$k = $v\n";
    }

    print("\n");

    while (my ($k, $v) = each %$opts) {
        print "$k = $v\n";
    }
}

my %opts = qw( a 1 b 2 c 3 d 4 );
scalar each %opts;
_split_options(\%opts);
[download]

a = 1
b = 2
d = 4

c = 3
a = 1
b = 2
d = 4
[download]

So no, it's not a bug in while, or even with each. It's a bug in your use of each. Whenever you use each, you should reset the iterator first. You can do that efficiently by calling keys in void context first.

sub _split_options {
    my ($opts) = @_;

    keys %$opts;  # Reset iterator
    while (my ($k, $v) = each %$opts) {
        print "$k = $v\n";
    }

    print("\n");

    #keys %$opts;  # Previous loop reset iterator
    while (my ($k, $v) = each %$opts) {
        print "$k = $v\n";
    }
}

my %opts = qw( a 1 b 2 c 3 d 4 );
scalar each %opts;
_split_options(\%opts);
[download]

c = 3
a = 1
b = 2
d = 4

c = 3
a = 1
b = 2
d = 4
[download]