The => operator aka the FAT COMMA is a synonym for a comma so Perl sees your code as:

$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' );
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The => is SYNTACTIC SUGAR to make hash assignments look spiffy.....It also lets you pass strinct and not bother to quote the LHS as shown

cheers

tachyon

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