I am not sure that I can help. current Pack docs. I am on Perl 5.24 and it appears to me that you are using a feature from Perl 5.30? This "D" option to pack() appears in red in my browser.

If Perl source code of: 

$ perl -le 'print unpack "h*", pack "D", sqrt 2;'
4846ed9f333f405bfff3000000000000

[download]
works? I am also unsure about what will happen on a different architecture. My main machine is Intel (little endian). I have a Raspberry PI which I think is big endian, but I have a lot of hardware cables to screw around with to get it working. Sorry. 
D  A float of long-double precision in native format.
         (Long doubles are available only if your system supports
          long double values _and_ if Perl has been compiled to
          support those.  Raises an exception otherwise.
          Note that there are different long double formats.)

h  A hex string (low nybble first).
H  A hex string (high nybble first).

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In reply to Re^3: [OT] Endianness and extended precision (80-bit) long doubles by Marshall
in thread [OT] Endianness and extended precision (80-bit) long doubles by syphilis

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