Long doubles are available only if your system supports long double values _and_ if Perl has been compiled to support those.

Yes - I ran the one liner on a build of perl-5.30.0 whose NV was long double.
Not sure how far back the "D" option goes, but it's available in 5.8.8 onwards, at least.

With recent perls we can ask for either little endian or big endian outputs - eg (on Windows):

C:\_32>perl -le "print unpack 'h*', pack 'D<', sqrt 2;"
4846ed9f333f405bfff3000000000000

C:\_32>perl -le "print unpack 'h*', pack 'D>', sqrt 2;"
000000000000f3ff5b403f339fed4648
[download]

However, I think the "big endian" output given here is simply the reverse of what was found on my machine.
I don't think there's any basis to assume that it displays what's stored on an actual big endian machine.

Cheers,
Rob