But when you're trying to (mis)use formatting of doubles in the way that List::Util::uniqnum() does, then you really do need a width of 19 decimal digits if ivsize is also 8.
I certainly sympathize. I've been trying to solve the same problem in List::Util::PP, and so far have settled on using roughly pack('FJ', $_, $_). There are still some issues around the unsigned int upper boundary though.
It should be possible to avoid the special case to use Gconvert by tweaking the format you use. I can get the full width output on my Ubuntu system by using '%0.19g':
$ perl -e'printf "%.19g\n", 2 ** 57'
$ perl -e'printf "%0.19g\n", 2 ** 57'