I am also very happy that you got this figured out! I very seldom work with floats.
In my investigations, I found this:

use Config;
# This should print 8 if on a 64 bit machine...
print $Config{nvsize},       " float size in bytes\n";
[download]

Might consider checking that as a sanity check that you are on 64 bit machine. I have no idea how to get Perl to do 64 bit floats on a 32 bit machine - but there is probably a way.