I suppose this is due to the insufficient precision in the NV representation

Yes, you're right - the precision has been lost. I was fooled by the fact that the following tests re $x (the IV) and $y (the NV) return "true":

$x == $y
$x - 1 == $y - 1
$x + 1 == $y + 1
[download]

I took that to indicate that perl knows that $y contains the value 144115188075868217, and that my problem was merely one of how to get at that value. The following tests also return "true":

$x == $y + 1
$x == $y - 1
[download]

which doesn't fit too well with the hypothesis that "no precision was lost" :-)

Thanks almut, for setting me straight.

Cheers,
Rob