$hash_codes{$2},$5

# thus the useless use of hash element in void context. 
# there is also a useless use of scalar in void context as well.

# you want
$hash_codes{$2} = $5

# this is normal usage of =>
%hash = ( key1 => 'val1', key2 => 'val2' );
# which parses the same as:
%hash = ( 'key1', 'val1', 'key2', 'val2' );