Think of a %hash as two arrays, @keys and @values. The number of elements in @keys is always equal to the number of elements in @values.

Storing in a hash $hash{$key} = $value; is like:

for ($i=0; $i < @keys; $i++) {
  if ($keys[$i] eq $key) {last}  # 'last' exits from the for loop
}
if ($i < @keys) {
  # $key already exists in the hash
  $values[$i] = $value;
} else {
  # $key is new
  push @keys, $key;
  push @values, $value;
}
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