local $\ = "\n";

my %h = map { $_ => 1 } qw( a b c d );

for my $pass (1..3) {
   print "Pass $pass:";
   while (my ($k,$v) = each %h) {
      print $k;
      last if $pass == 1;
   }
}

##</code><code>##

Pass 1:
c
Pass 2:
a          <---- 'c' missing
b
d
Pass 3:
c
a
b
d

##</code><code>##

local $\ = "\n";

my %h = map { $_ => 1 } qw( a b c d );

for my $pass (1..3) {
   print "Pass $pass:";
   keys %h;  # Reset iterator
   while (my ($k,$v) = each %h) {
      print $k;
      last if $pass == 1;
   }
}

##</code><code>##

Pass 1:
c
Pass 2:
c          <---- Welcome back
a
b
d
Pass 3:
c
a
b
d