Your very attempt to limit $checksum to 32-bits is causing an overflow. & forces its arguments into native ints, and the pre-limiting value of $checksum doesn't fit into one. 
$ perl -le'print $ARGV[0] & 0xFFFFFFFF' 1000000000
1000000000

$ perl -le'print $ARGV[0] & 0xFFFFFFFF' 10000000000
4294967295

[download]

Use a floating point operation.

Replace
$checksum = $checksum & 0xFFFFFFFF;
with
$checksum %= 2**32;

In reply to Re: Difficulty restraining read function to 32 bits by ikegami
in thread Difficulty restraining read function to 32 bits by TeamViterbi

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