for my $k (@keys_existing_in_both) {
    my $exec = $hash2{$k};
    # do something with $k
    my $result = md5($k);
    if ($result ne $hash2{$k}){
         print "Hash sum miss match for '$k'!\n";
    }
}
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(BTW in the general case exists $hash{$key} checks if an key exists in a hash, not defined $hash{$key}.)