I'm not sure if this helps ...

Heh ... it helps ... but only in that it reaffirms what I already knew ;-)

I'll elaborate a little on how this all came about.
As I mentioned in the opening post, with perl-5.34.0, pack's 'D' templates are allowed on all builds irrespective of whether $Config{nvtype} is 'double' or 'long double' or '__float128'.
In earlier perl versions the 'D' templates were supported only if $Config{nvtype} was 'long double'.

With perl-5.34.0 and nvtype of 'long double' we get:

C:\>perl -wle "print unpack 'H*', pack 'D', 2.4;"
9a999999999999990040000000000000
[download]

whereas with perl-5.34.0 and nvtype of double, the same command yields a slightly different result:

C:\>perl -wle "print unpack 'H*', pack 'D', 2.4;"
00989999999999990040000000000000
[download]

That's pretty much as I expected because the string being unpacked in the first one-liner is different to the string being unpacked in the second one-liner.
But then I wanted to see what the result would be if the string created on the long double build (in the first one-liner) was fed to the 'double' build (in the second one-liner).
Would the result be 9a999999999999990040000000000000 or 00989999999999990040000000000000 ?
(I was quite sure it would be the former, but I knew that if I didn't check then I'd get it wrong.)

So I thought I'd do something cute:
I decided that, on my 'long double' build, I would run the following script:

use warnings;

$template = 'D<';
$nv = 2.4;
$p = pack $template, $nv;

$s = "'$p'";

# Now pass $p to the 'double' build of perl.
$double_perl = "C:/perl-5.34.0/bin/MSWin32-x64-multi-thread/perl.exe";
system $double_perl, '-wle', "print unpack('H*', $s);";
[download]

But, as already demonstrated, that doesn't DWIM.

In the end, I took the clunky approach of running 2 scripts.
Firstly, on the 'long double' build of perl-5.34.0 I ran the following script that would print the string returned by pack() to a file:

use warnings;

$p = pack("D", 2.4);

open WR, '>', 'packstr.txt' or die "Opening: $!";
binmode(WR);
print WR $p;
close WR or die "Closing: $!";
[download]

Then, on the 'double' build of perl-5.34.0, I ran a script that passed the string that was saved in packstr.txt to unpack():

use warnings;

open RD, '<', 'packstr.txt' or die "Opening: $!";
binmode(RD);
$p = <RD>;
close RD or die "Closing: $!";

print unpack("H*", $p);
[download]

That script output 9a999999999999990040000000000000.
(Hmph ... as if it was ever going to do anything else ...)

It just would have been so much cleaner and simpler if I could have got the first approach to work.

Cheers,
Rob