It's not what you want, but an ordinary hash behaves the same way:

my %h;
@h{a..d}=1..4;
while (my ($k,$v) = each %h) { last }
while (my $k = each %h) { print $k }

__END__
abd
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The each call for the print loop only finds three keys. You have to manually reset the each iterator if you do not read all pairs or keys. That's typically done with keys %h; in void context.