Do pointless things more elegantly using vec:

...
while (my $blk = <$fh>) {
    for(my $bit_idx = 0; $bit_idx < length($blk)*8; ++$bit_idx) {
        my $bit = vec($blk, $bit_idx, 1);
        if ($bit != 0 && $bit != 1) {
            # Error & warn ...
        }
    }
}
...
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