my %hash = (a => 1, b => 2);

my %seen_key = ();
my $seen_key_count;

do {
	$seen_key_count = keys %seen_key;
	foreach my $key (keys %hash) {
		# skip any entries deleted since we called keys
		next unless exists $hash{$key};
		
		# skip any keys we have already seen
		next if $seen_key{$key}++;
		
		my $value = $hash{$key};
		
		# do stuff with $value
		print "$key = $value\n";
		# and mangle the hash
		$hash{c}="foo";
		delete $hash{b};
	};
} until ($seen_key_count == keys %seen_key);