Well... I don't know what the problem may be, but maybe you should add a

print length($hline),"\n";
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before the substr that's dying, just to be sure, maybe even print the line.

But the main reason I write is to suggest using one unpack instead of three substrs... syntax would be (approximately, not tested)

($xh,$yh,$zh) = unpack("x31A8x1A8A8",$hline);
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Pretty sure that would be more efficient.