if ($device eq undef) {
        die "Usage: <app> /dev/hidrawN";
        exit;
}
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undef is neither a string nor a number.    You have to use the defined function to test whether a variable is defined or not.    Or you could just test whether @ARGV was empty or not.    die exits from the program so using exit there is superfluous.

@ARGV or die "Usage: <app> /dev/hidrawN";
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    my $report = ord(substr($data, 1, 1));
    my $status = ord(substr($data, 2, 1));
    my $unit   = ord(substr($data, 3, 1));
    my $exp    = ord(substr($data, 4, 1));
    my $lsb    = ord(substr($data, 5, 1));
    my $msb    = ord(substr($data, 6, 1));

    my $weight = ($msb * 255 + $lsb) / 10;
    if($exp != 255 && $exp != 0) {
        $weight ^= $exp;
    }
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If you used unpack you could extract all your data in one statement and you wouldn't have to calculate the $weight value yourself, which is calculated wrong because you should be multiplying by 256, not 255.

    my ( $report, $status, $unit, $exp, $weight ) = unpack 'xCCCCS', $
+data
    $weight /= 10;
    if ( $exp != 255 && $exp != 0 ) {
        $weight ^= $exp;
    }
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