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sub BigHex{ 
    my ($int,$long) = unpack'nN', pack 'H*', $_[0]; 
    return 2**32 * $int + $long 
}

print BigHex 'ffffffffffff'
281474976710655

print BigHex('ffff0000ffff') | BigHex('0000ffff0000')
4294967295

use constant xFFFFFFFFFFFE => 281474976710654;