Run the following, and tell us what the output is:

$preprod->{$siebdb} = $k;
print "[$k]\n";
$prod->{$siebdb} = $j;
print "[$j]\n";

if ( $preprod->{$siebdb} eq $prod->{$siebdb} ) {
    print "SAME\n"
} else {
    print "DIFFERENT\n"
}
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