As to the floating point number:

my $hex = "403b8811bb1366e4";
print unpack "d", reverse pack "H*", $hex;  # 27.53152055 (on x86 arch
+)
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but, as moritz said, binary representations of numbers are machine dependent... (except single-byte ints), so depending on the architecture that you're running this on, you may get wrong results (note that "d" is "A double-precision float in the native format").   (Update: in practice, however, most architectures these days use IEEE 754 format, so you essentially only have to figure out whether little- or big-endian representation is being used — handled by the reverse in the above snippet.)

Also, in case you're reading the data from the file in binary (not as a hex string), you of course don't need the pack "H*" part shown above.