But you do have to worry if you for some reason call 'each' or 'keys' on a hash in an inner loop of an each loop. There is not really any way around this expect to use 'keys' in your outer loop instead of 'each'.

# this code creates an endless loop
my %hash = (
  foo=>"x",
  Foo=>"y",
  Bar=>"a",
  bar=>"c"
);
while(my($key,$value)=each %hash){
   # dumb example 
   for my $key2 (keys %hash){
       if(lc($key) eq lc($key2)){
         print "key $key is similiar to $key2\n";
       }
   }
}
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