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

##</code><code>##

print BigHex 'ffffffffffff'
281474976710655

##</code><code>##

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

##</code><code>##

use constant xFFFFFFFFFFFE => 281474976710654;