$inc += 0;
$exp += 0;
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That's why you get this warning, because Perl can't know what your real intentions are.

Please note how accurate the warning is, it gives you the data line and the code line...

Update

°) I just realized that since you are using unpack you get an empty string not undef (like a regex would).

Hence adding 0 will continue giving you a warning.

So better use the $inc ||= 0 approach here, like AnoMonk suggested