Anonymonk beat me to the punch. But, the reason for why foreach (keys) and while (each) behave differently has nothing to do with keys being an iterator or not. (Well, it does, but not really.) It has to do with the difference in behavior between foreach and while. foreach is defined to operate on a list. If you give it a list, then you're good. If, however, you give it a function or keyword, then it has to call that function/keyword and construct a temporary list with the return value(s). Incidentally, this is why the following doesn't DWIM:

my @x = 1 .. 5;
sub get_x { @x }
foreach my $v ( get_x() ) {
    $v *= 2;
}
print "@x\n";
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while, however, re-evaluates its condition every time. This is why the while-loop goes infinite, but the foreach-loop doesn't.

while (my ($k, $v) = each %hash) {
    $hash{ $k . 'a' } = 1;
}

foreach my $k (keys %hash) {
    $hash{ $k . 'a' } = 1;
}
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