Every hash variable in Perl has its own internal iterator, accessed by the builtin 'each', 'keys', and 'values' functions. The iterator is also implicitly used whenever the hash is evaluated in list context. The iterator is "reset" whenever 'keys' or 'values' is called on a hash, including the implicit calls when the hash is evaluated in list context. That makes it dangerous to do certain hash operations inside a 'while ... each' loop:

while (my($k,$v) = each %hash) {
...
@k = sort keys %hash; # Infinite loop!
@v = grep { /foo/ }, values %hash; # Ack!
print join ' ', %hash; # Run away!
}

'Hash::SafeKeys' provides alternate functions to access the keys, values, or entire contents of a hash in a way that does not reset the iterator, making them safe to use in such contexts:

while (my($k,$v) = each %hash) {
...
@k = sort safekeys %hash; # Can do
@v = grep { /foo/ }, safevalues %hash; # No problem
print join ' ', safecopy %hash; # Right away, sir
}