sub make_hash_iterator {
   my ($hash) = @_;
   my @keys = keys %hash;
   return sub {
      return () if !@keys;
      if (wantarray) {
         my $key = shift(@keys);
         return ($key, $hash->{$key});
      } else {
         $_[0] = shift(@keys);
         return 1;
      }
   };
}

my $i = make_hash_iterator(\%hash);
while ($i->(my $key)) {                # scalar context
   ...
}

my $i = make_hash_iterator(\%hash);
while (my ($key) = $i->()) {           # list context
   ...                                 # Will fail for '' and '0'
}                                      # without parens around $key.

my $i = make_hash_iterator(\%hash);
while (my ($key, $val) = $i->()) {
   ...
}