In my case, this was the correct answer. I needed to switch from big to little-endian. It's amazing how little there is on the internet on this subject. Hopefully this will help future users. 

my $n = pack 'H16', '3f0df9673344c570';;

printf "Big\n";
print unpack ("d", scalar reverse $n);

#Big
#5.71713084433029e-05

printf "\nLittle\n";
print unpack ("d",$n);
printf "\n";

#Little
#1.69044404965372e+235

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In reply to Re^2: Converting binary to double precision (big-endian) by ahoriuch
in thread Converting binary to double precision by ahoriuch

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