As saouq correctly points out there is no difference between:

for my $key (keys %my_hash) {
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and

for my $key (my @temp = keys %my_hash) {
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Both create a temporary list. Both will be unaffected by changes to %my_hash.

If you want to make sure you cover keys added in the loop processing, and skip keys deleted by the loop processing, then you'll need to do something messy like:

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);
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But I'd agree with jarich and dragonchild. This smells like a design problem. If I needed to code anything like this I would have a very strong suspicion that there was something seriously wrong somewhere in the codebase. Rather than fix the symptom I would try and track down and eliminate the disease.

Could you give more of a description why this is necessary? Perhaps we could suggest an alternative solution.