##
1..1
not ok 1 - they are the same
# Failed test 'they are the same'
# at w.pl line 40.
# 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));