Ok. Here's what seems to be happening in these two statements:

• 1. In the while loop, the pairs of Keys and Values from the hash %by_key are being fetched.
• 2. First things first: $key_list_by_value{$value}.key_list_by_value is a hash variable, which appears to contain array-refs, hence the use of @{ } around this variable.
• 3. What this second line of code is doing is that it is adding the current $key to the list pointed to by the current array-ref

Here's the understandable version of this code:

while(($key,$value)=each(%by_key)
{
   $current_key_list_aref = $key_list_by_value{$value};
   push(@$current_key_list_aref,$key);
}
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