use constant CAN_PACK_QUADS => !! eval { my $f = pack 'q'; 1 };
use Math::BigInt;
print quad('ffffffffffff'), "\n";
sub quad { 
    CAN_PACK_QUADS
    ? unpack( 'q<', $_[0] )
    : Math::BigInt->new( '0x'.$_[0] )
}
__END__