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On my Ubuntu system, it is using sprintf: 
$ perl -V:d_Gconvert
d_Gconvert='gcvt((x),(n),(b))';

1..1
not ok 1 - they are the same
...
#          got: '9.2233720368547758e+18'
#     expected: '9223372036854775808'
# Looks like you failed 1 test of 1.

sv_setpvf(keysv, "%.19g", SvNV(arg));
is, in effect, converted to:
sv_setpvf(keysv, "%.17g", SvNV(arg));